I have tried a bubble that fits to the rail & one that fits onto the scope mounts... both total rubbish, the scope mount one relies on you having the mounts clamped equally on both sides & the rail one once tightened went off. I had both on at the same time & they both said different things !!
Now I stick one on the top of my scope turret & use the plumb line method. Sometimes I will check with one flat on top of the rail as well.
In life you should not persue goals that are easily achieved, develop an instinct for what can be only just achieved through your greatest efforts.