Scope base or receiver leveling?

[WEATHERBY460]

On my Savage 110, is it more accurate to level a scope to the base or to the receiver where there is a flat spot towards the rear of the gun where the bolt goes in? I thought these 2 points would be square to each other...but there not on this gun...thanks

 

[woods]

Vertical Retical Instrument

EXD ENGINEERING VERTICAL RETICLE INSTRUMENT - Brownells

 

[Coyboy]

I level them on the rear tang flat, It's hard to trust the scope rail as level when bolted to a factory and somtimes custom reciever.