Its not very far at all. If its a threaded assembly, its probably only an inch of rotation inwards. I've done this on cheap centrefire scopes to accomodate rimfire distance. On the Bushnell I experimented with, the front lense assembly is well sealed with grease, and a small turn of the threads wouldn't let moist air in. But, you proceed at your own risk.
If you are going to do it, first figure out how to get it loose. Then, set the gun and scope up on a rest at a target at 100 yard. Move your head around and watch the image move under the crosshairs. Make Very small corrections to the front lense and watch the effect. Find the right position, mark it, and lock it down with the locking ring (if that's how its made on your scope). Don't turn it far, it shouldnt take much to change it from 50 to 100.
