Hi Judith,
I'm using a cheap 8mm (about 3/8" ) drill bit for concrete and stone and switch my drill to hammering mode. To protect my plates I place some bubble foil under it and fill the plate about 5 mm ( 1/4") with water for cooling. I always drill from the inside as the first hole tends to break out irregularly on the backside. Then I drill some more holes around the first one and end up with a hole of about 4 cm diameter (1.5") that is sufficient for drainage. As I rarely look at my plates from below I do not care if the hole is not completely round...
