Well I'll probably use the rubber gasket as my guide. The mounting screws are small but there are protruding posts that go into the shell so I have to drill holes that will be big enough for them to fit into. I don't want to screw this up.
Use the gasket as a guide, absolutely. I recently installed two bass drum cymbal mounts. Always start with a small pilot hole (2 mil or so) and work up to the size you need. Never drill the full size straight away - that's a recipe for disaster. If you're not sure about the size you need, just drill progressively and try to fit the part each time.