Gabel has the right questions, but you might be dealing with this.
Most pitches are going to give you one cut (either the level or plumb) that your saw is not going to be able to make. I usually cut these on their backs or tops (whatever you want to call it).
In this picture it is a 10/12 which leaves the level cut at 50.19. Some saws wont make it especially 8/12 and lower (or 14/12 and steeper)
As you know it is pretty damn difficult to run an angled beam saw across a 4X. This set up is even more difficult but helpful. If you can cut with precision this way you don't even half to clean up with the chisel afterwards(if your rafters are exactly the same dimension in depth).
If you are looking for speed you might want to look into this. Create a angled block to attach to your beam saw base. Whatever the angle is of the block you can add to the maxed out limit of your 45 deg on your beam saw. Attach to the saw add some wax and be prepared to be dog tired from concentration afterwards .
*This is effecient if all your rafters are very close to the same depth. You can stay a little away from the corner and then a couple whacks with the chisel will clean it right up and save you from handsawing or having to flip 80 rafters (that would be necessary if you cut the rafters on their sides).
And if they are all the same dimension you might be able to gang cut them side by side. I usually stay away from this though, it can get hairy and sloppy.
80 Rafters might just be enough for this.