I'm sending a 16 inch barrel to be chopped and threaded for use on my SBR.  Any opinions on "best" length?

Depends what the goal of the rifle is. For an all-purpose rifle, 12.5" seems to be the sweet spot for 7.62x39. It loses little velocity (certainly not enough to bring it below the velocity threshold of modern defensive ammo) and as such can do everything the 16" carbines can, but have better balance and handling. Adding a can brings you out to normal carbine length instead of out to musket length. If you're going for a PDW style weapon, I would emulate the 8.5" Arsenal krink's front end with the folding stock of your choice. You'll still get useful velocities out of the 8.5" barrel (most of the powder in most loads has already burned by then), even if your effective range is less than that of the 12.5" and longer barrels.

After a moment of indecision I am going to go with 11.5.  My suppressor is rated for 10+, I've already got a 10.5, and I didn't feel like 12.5 was worth the tax stamp.  11.5 seemed like a good compromise.

12.5 on paper doesn't seem like much, but in the AK it really is the perfect length. Balance is perfect, and like stated above the loss of velocity is minimal.

7.5 inches because you can just buy a mini Draco.  You can SBR it and suppress it.

My AR and AK will both be 12.x" in 762x39

Still waiting on my 11.5 A2 FSB upper to get back.  Fired my 10.5 inch for the first time this weekend and it's a really cool gun.  Tried it without suppressor (just because) but with an ASR MB and it was violent & loud.  Blew a bunch of stuff off the shooting bench to the left and right

With a suppressor is