Ooh, eck, I've got all this to come, haven't I?
Luckily, my controller is so old, it doesn't have a tool height setting program; I do have a rather accurate handwheel thingy though, so I can manually set tool heights without having to worry about overtravel.
How repeatable are microswitches? I'm guessing they must be reasonably repeatable as my machine uses them as both limit switches, and reference marks, when setting itself up. It had occurred to me if they were good to, say, 0.01mm, they'd make an ideal tool height setter without having to worry about conductivity.