Your cross hairs? If you can't look through your scope and tell if your rifle is canted or not I'm not sure a bubble level is going to help. Ya you might not be perfectly level using your cross hairs but close enough for all put the longest shots. Unless you have a dead solid rest the little movement of your head from looking at the level back to your scope can shift the cant so why not just use the cross hairs in the scope.
Your cross hairs? If you can't look through your scope and tell if your rifle is canted or not I'm not sure a bubble level is going to help. Ya you might not be perfectly level using your cross hairs but close enough for all put the longest shots. Unless you have a dead solid rest the little movement of your head from looking at the level back to your scope can shift the cant so why not just use the cross hairs in the scope.