I am looking to mount a 20X SS scope on a Barrett M99.   I have some Burris Xtreme Tactical mounts laying around that I thought would be suitable.  What tips would you guys give.  How tight do I snug things down, What is the minimum eye relief you would recommend, ect...
Thanks

You cannot set the eye relief, whatever eye relief you have came from when the factory built the scope. Follow the Mfg. scope mounting instruction for proper torque settings on the ring screws and the crossbolts that hold it to the scope mount rail.

What you can do as far as eye relief is put the scope in the rings loose, get down behind it in a shooting position that's comfortable. Adjust the scope so that you can see through it w/o any shading or distortion in the scope, close your eye and open it a couple of times after doing this to make sure it's clear. If it looks good then snug it down to keep it from moving, then follow the Mfg's. pattern for tightening and torquing the screws.

"For now" I have a Bushnell 3200 on my Steyr 50. I believe that the eye relief is 3 inches.
The muzzle brake tames the majority of the recoil, and you get "pushed" with the rifle, equaling things out. You won't get your eye poked out with that thing!
But the big 5-0 is a mighty Beast so I am glad you are concerned with erring on the side of safety.
The first responder to your post? "What he said."