Pardon me if I'm overlooking something obvious, but I don't think simply using the same drill bit can guarantee that all holes you drill will be the same size.
If the drill bit goes through the cap at an angle instead of perpendicularly, the hole will be oval instead of circular, right? It will be a larger opening, and the area of the opening will depend on the precise angle.
If you're doing these by hand, it seems like the same bit could yield holes of slightly different sizes and shape. The difference may be negligible if you have a steady hand; I don't know. I just wanted to bring up the point.