Without a drill press, accurate control of hole depth with an electric drill is difficult to achieve, at best.

Drill a hole in a short length of dowel that's large enough in diameter that it won't split and fits snugly on the bit. To adjust for the depth of the hole, vary the dowel's length and the placement of the bit in the chuck. You can make several of these for a variety of depths and bit sizes to keep on hand, or toss them when finished.
—From the WOOD® magazine shop