I am building my first AR and looking at mounts for a scope. I have it narrowed down to two scopes, each with about 3.6" eye relief. Given an a4 upper, a standard a2 butt stock, how far forward of an offset should I be looking at? I am looking at an American Defense mount, and they come with either no offset, or a 2" offset. I presume I will need to get a 2" offset for optimal scope position with the larger eye relief?

Thanks in advance.

get a cantilever type. You can always move it back on the flattop. The other way around your shit out of luck!

In the world of the AR15 you will never see as many scope mounted variances. Height, and position forward and back.

All my scopes eye pieces are 3/4" to 2"+ ahead of the charging handle with an A2 stock depending on the scopes eye relief.

I like to have my scope mounted so that my eye is in the same place whether using the scope or the BUIS.  For different rifles (20" HBAR clone vs. 16" carbine) with different scopes I've found I can't use the same mounts on each rifle.  So each of my rifles has ended up with a different mount.

I've had to experiment with mounts/risers to get the scope position I want.