I have a B square fitted to my .22lr and it is excellent, particularly if you shoot on steep ground. You will be suprised how much you will cant your rifle. Also good when taking 100+yd rested shots. The B sqare attaches directly to the dovetails. Sportsmatch do ?2 versions and swing out ones are also available. I have also seen them built into the mounts. If you ever come across a Kasnar Beta 3 scope they have an intergral spirit level built into the scope. Also excellent.
If you do alot of shooting on hilly ground or air rifle for squirrels then a Cosine indicator is an underated piece of kit, and works v well with a spirit level.
D