I'm about 4" left off center at 25 yards which puts me all the way to the right side of the total windage on an A1 sight.  I'm thinking of canting the FSB to the left to make up the difference and get the rear peep centered.  Anyway, I'm guessing I'll need about .026" at the index pin to accomplish that.  Sound about right?

Read my thread and the link to the thread within.  Makes a difference whether you clamp barrel or receiver.  Big difference!here

I don't use a barrel clamp although I have on occasion.  I generally use a Geissele reaction rod but this time I used a Brownells receiver clamp.  I don't have any wiggle room at the index pin so I don't think any amount of torque will get the barrel rotated where I need it.  I've seen some pretty sloppy index pin slots so I think opening it up a tad and rotating the barrel in that direction is my only solution.

In any case, my calculation goes something like this.  I'm 4" off at 25 yards and my sight radius is 20".  So 4" is to 25 yards as x is to 20", x being 80/900 or .089".  That's the distance I need the front sight to be moved.  I just realized I calc'd that wrong earlier so I'm glad I'm writing this down.  The sight is at 2.5" radius and the index pin is at .5" radius.  The circumference at 2.5" is 15.708" and 3.142" at the index pin.  So .089 is to 15.708 as x is to 3.142, x being .018".  Not as bad as I was thinking.  I'm just going to feeler gauge the index pin slot and add .018" to the side I need to move the base.   I don't think the arcs are going to make that big a difference so I'm not really worried about the wink in the arc.  

I'll report back how much I screwed things up.  HA!  The sight radius is actually 19.75" for those who noticed, doesn't change anything for my purposes.

have you thought about taking the pinned FSB off and replacing it with a clamp on triangle FSB?
its always a option if you cant bring the pinned FSB around.

yikes, forgot this was the retro forum, well you could pass it off as a tool room special.

Wait a minute, I've seen this movie......

For those interested, the .018" adjustment was near perfect.  Moved it over almost exactly 4" at 25 yards and now the peep is nicely centered.

The procedure was simple.  I took a stack of feeler gauges until they fit perfectly in the index pin slot.  I then added the .018 gauge to the stack and taped them together.  Took a flat micro file and worked on the side I wanted the sight canted to, testing with the gauge stack every 15-20 seconds of filing.  Within a few minutes I had the stack fitting perfectly in the slot.  Then it was just a matter of rotating the index pin to the expanded side and torquing the barrel nut.

Got to love it when a plan works and now I have full windage adjustment with very little effort.   Plus it just looks nice centered.  

Was the front sight a bit off to begin with?  Seems odd that you used up all your adjustment.
Glad it came out for ya'; this retro stuff is fun.
Moon

Yes, it was a new manufacture 20" barrel on a very good condition late 604 arsenal refurb.  Everything is tight and the receiver is square.  I am not impressed with this manufacturers ability to install a FSB.  Maybe they had a bad day.  In any case, it's fixed, with the help of a little 8th grade algebra.  

I haven't read many (I can't think of any) threads talking about adjusting the index pin slot to account for a canted base.  Doesn't seem like too much interest.

Time will tell if it wanders, but I'm 99% sure it's going no where. If it does, I'll just glue the barrel or JB Weld the gap so it cant move.

A few years back, I had an SP1 upper that I actually had to relief the left side of the notch in the upper receiver  (the way you described) to rotate the barrel and FSB on a Colt "CMP CHROME BORE" FSB to center rear sight. I peened the threads on the opposite side to prevent it from rotating back.  That was a Colt factory upper set up.  It happens.  It just takes the upper being a little off and the index pin or FSB to be a little off.  Add it up and it becomes a problem.

In the recent case of my AR15 Sport pencil barrel, I assumed it was the barrel because the DSArms M4 barrel was perfectly centered in the upper.  As it turned out, when I pulled the AR15 Sport barrel, the index pin and FSB were dead nuts on.  even though it was a minty 604 upper, there was a almost imperceptible amount of wiggle.  Apparently that wiggle was enough to cause a different to the angle of the FSB depending on whether barrel or receiver was clamped. When I clamped receiver instead of barrel, it moved the FSB at least 1/16 or a little more to the left, centering it nicely.

Yeah, I guess I shouldn't be too shocked these things can happen since I moved mine a little under a 50th of an inch.  Doesn't take much.  

Mine moved the sight/FSB a little more than 1/16", so probably close to what you moved your index pin.  The sight is probably 5X the distance to center of bore axis as the index pin.

I haven't read many (I can't think of any) threads talking about adjusting the index pin slot to account for a canted base. Doesn't seem like too much interest.

Basically doesn't happen like talking about it and most here know how to remedy if necessary.  It's been discussed in detail about every year and a half.  Usually happens to three in one week then nothing for a couple years.  I carefully hand ream taper pin holes personally if necessary and make adjustment to pins and FSB if torque and position remain the same or easily duplicated.  The machinist simply offset the index pin or many simply shim if possible.  Kinda depends on the skills one has / funding / tools as to how they go about repair.

Only had to ream slots on a couple Sarco barrels sold as screwed up for $85ea. and a couple for friends who bought canted barrels.  Really never had but one or two I couldn't fix with a torque wrench holding back or pushing while applying torque.  Been lucky I guess.  Lots of ways to repair depending on actual cause.  Glad your algebra worked out for you.  I would never thought about using the mathematician skills. Should have seen me adjusting a clamp on national match sight the other day.  Wasted 20 rds before I came to my senses.  Trying to do it without reference point.  Was funny looking back.  Now hits 3in steel plates at 150-200 yds with near mechanical zero until I can't see them around dusk.  I'm gonna drill my hooded aperture out a tad bigger like 3/64" to start.  Then have to duplicate situation etc.

Ive been there! Had to shim my index notch!

https://www.ar15.com/archive/topic.html?b=3&f=123&t=603748