Keep in mind that there are two separate parts, the bench or table top if you prefer and the rest that the gun rests on. A typical bench will be fixed in place and the distance to the target is measured from the front edge of the bench. Typically this is also the same distance as the cement pad that the bench sits on. The rest can be anywhere on the bench, however for safety sake most range masters will require that the end of the muzzle be close to or over hanging the forward/front edge of the bench.
Some ranges use a "firing line" that is marked/painted on a cement pad. The front edge of a bench would be aligned with the firing line and target distances marked from the firing line.