I have decent little construction level, use Harris benchrest shorty swivel bipod and tighten the heck out of the swivel, use a little v-notch rear bag and level rifle on kitchen table, doesn't move if careful setting scope in rings and then hanging on wall I have some flo orange paracord hanging from a nail with a box wrench hanging on other end and I just move the rifle/scope on kitchen table so crosshair is pretty much along side or inside the cord, then lift scope off and confirm rifle still level, set scope back on and level to the plum line and watch as you snug the rings down you keep it level, the base of rings usually already torqued close to spec and eye relief ring spacing etc. already preset/marked. It's pretty crude but works well for me.
Then it's easy to clamp on scope tube bubble level and keep scope lined up on the plumb while you set that up to show level...if you add a scope tube clamped bubble level. Do the whole thing in kitchen off the table. I used to just do same to level rifle then level off top off turret but seen enough reticles not exactly plum to turret top so now just use the hanging paracord and so much happier with system. The fat paracord in bright color at pretty close range makes it easy to see reticle inside or play with magnification to get fine lines of orange on either side top to bottom of reticle.