I want to compare ballistics charts between subsonic vs supersonic loads (300 blk) using the same zero.

In other words: if I were to zero with supers at 100 yards, then I need to figure out where my zero would be using subs.

The only way I can think of to do this would be to use the first 2 ballistic entries to compute the slope, then experiment with different zeros on the other load until I get the same slope using the first 2 ballistic entries on that load. But this would be time consuming and not all that accurate.

Is there an easier way?

Mine is 7 inches lower. Sight in with supers at 100, shoot subs and see where they land. In my case supers were 150gr Interlocks at 1750, subs were 203cast at 1050.

Assuming you already have a supersonic zero, why not just take some subs to the range and shoot to find the subsonic zero?  I shoot both subs and supers from a .308 win.  Bullet diameter is the same and weight (220 grains) is probably very similar to what you are using.  From my supersonic zero, I have to dial up ~3.5 mils @ 100 yards to get to my subsonic zero.  For 300 yards, I have to dial up 12.5 mils using my supersonic zero as a starting point or up 9.0 mils from my subsonic zero.  You are probably looking at less of an elevation difference that me because my supersonic rounds are going faster than yours.

Having said that, to compare the ballistic charts of your subsonic and supersonic loads, you don't need to zero at all...  just put the data for each round into a ballistic calculator.  It will give you a chart for each at whatever zero range you choose.  To bring that data into real world relevance, all you have to do then is find the subsonic zero for your rifle.

You can do it in JBM.  Solve for your supers zero range and note the elevation angle (between bore and line of sight) when you solve with the Elevation Correction for Zero Range checked.  When you input yours subs data you input that sight elevation angle in MOA in the box located in the area for sight height and other sight info, and then don’t check the box labeled Elevation Correction for Zero Range.  The solver will show trajectory from your line of sight angle that was solved when you ran your conditions for supers.  Sight height over bore is a critical input variable.  Don’t be surprised if subs never cross the line of sight from any reasonable supers zero range.

You can do it with AB's ballistic program that they install on the kilo2400's..It is setup to input offsets for different loads in the same gun...

The easy way to do it, is zero one, then without moving anything, shoot the other load, then record the corrections in the program and it will adjust automatically for the offset...I don't bother on my ELR hunting rifle..I zeroed it on my 230 grain Berger long range load, then just shot my 200 grain Barnes up close load on the same target...I will only use the barnes load at under 300 yards so just knowing where it is a 100 allows me to just shoot it..

This is both loads on 1 target...

The yellow is a 5 shot group of the 230 Bergers, the green is 5 shots of the 200 Barnes...

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Here is what it looks like on the kilo 2400.....it's done with the zero height/offset...

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I agree. Zero super. Find sub offset. Just make a chart with a normal Zero and add in the drop at Zero range to every range.

For example if you drop is 7 MOA and you have more drop at further distances just add 7 MOA:

0 + 7 = 7

2 + 7 = 9

6 + 7 = 13

Easy as that.

My 300 BO broke so I don't have any info on heavy subs....

But my 762x39 bolt gun works fine.  

"Normal" load being a 125gr TNT at 2300fps-ish, sub load being a 85gr .312" XTP at 1000fps

(speer makes .308 and .309 diameter 90gr bullets, or you can play with cast lead, but little light bullet subs are giggle inducing even if you don't have a can.  can cause feeding issues though 'cause they are short-n-stubby)

Sub load hit 9" low at 50 yards compared to the super....

But.... there are a few ways to make it all work.

Assuming you have a scope that tracks well, has sufficient adjustment, and does a great job on the box test, all you need to do is spend some range time and build a dope sheet, with a reference at reasonable divisions on how much needs to change to re-zero between the loads.  

Or in the finest enabler tradition of internet forums - have a "subs" gun and a "supers" gun.

FWIW I zeroed with supers at 36 yards.  Subs seemed to impact 1 MOAish below the supers. Easy hits on ipsc plate at 100 with both.