[dsraven]

depending on the bit, I have wrapped tape at the correct depth, used a paint marker, a nylon cable tie and even a hose clamp wrapped around the the bit. you could also make a rod that clamps to the drill so you can only get so close to the project or use a short section of abs pipe that fits over the drill bit and butts against the front of the drill, cut to length so the drill bottoms on the pipe at the correct drill bet depth.
where there is a will there is a way, lol.