There is no hard & fast formula. How much material to leave depends upon the type of material, hardness of it, the size and tolerance of the finished hole, and the reamer to be used.
When you say you have to make a hole for a 1/2" dowel pin, what type of fit do you require? Easy slip-through, snug-sliding or press fit? Most hardened dowels are toleranced as 0/+.0002".
What machine are you using to do this?