I need to pre cut my rafters for a strait gable roof on a pole barn.
Trial and error would be a bad option as it is just me with no helpers and it is in the air pretty far.
So I hope to pre cut two, drag them up there, check fit on each gable end and in the middle of the run a few times, then use those patterns and pre cut the whole run.

I forgot how to calculate the length from top long point of rafter down to inside 90 of the "Bird Mouth?".  Eave length will not matter.  I will pick a length based on remainder of board.

Ridge will be 1.5" wide (2X8)
Outside dimension on wall plates is 18'-4" or 220".
I need this figured for both 3.5 in 12 pitch and also for 4 in 12 pitch.  (target is 4 in 12 but I am close on lumber length so 3.5 in 12 is the fall back point)

Thanks all.

I think I am remembering some of it.

I take 220" and divide in half = 110".

4/12 = x/110" = 36.66"   (and this step is the key to figuring out 3.5 in 12 pitch as well?)

Pythagorean Theorem with 110" for A and 36.66" for B.

C= 115.95".

Multiply 0.95 by 16 for 1/16ths".

Common length = 115 - 15/16".  or 116" close enough.

Knock back 3/4" squared off the ridge cut/line AFTER measuring the 116" common measurement.

Provided I stay on the 4 in 12 plumb line down at the bird mouth end of the common measurement can I arbitrarily "pick" my up and down height to best suit the situation?

You don't have a blue book? Scratching head...let me see if I can remember this.  Find your total rise and the you use a little pythagorean theorum:

total rise squared + run squared = square of c (c = your rafter length). Get the root and then subtract .75" to account for half the ridge.

I think that's right, but man it's been a long time since I learned that.

eta: try this and see how she does for you: http://www.wendrickstruss.com/Calculator/index.asp

Hey Thanks!  That's a pretty good truss calculator.
No blue book.  Lost it to the great tool eating storage shed.  It is in there some place but dang if I can find it!

I think I got what I need to go ahead and pre cut the rafters.  
If I'm off a little the critters around here probably won't care anyway!
And of course lumber does grow in trees!  It's the milling that gets pricey.