Yes, thanks Andy, that's my usual method too. But it would be good to have
a reliable, universal procedure that resulted in getting it right first
time ;-)
I reckon the following seems logical and I'll try it next time:
1. Drill the hole (following guidance if there is any).
2. Use a hand vacuum to suck out the remaining dust.
3. Measure the actual diameter by inserting known drill bits until one
fits snugly.
4. Use a plug whose widest diameter (apart from any 'lip' at the end)
matches the hole. I have a micrometer, so should be able to measure that.
5. Use a screw whose widest diameter is X mm smaller than the hole. But
what should be the value of X? And would this vary with the type of plug?
6. Try it - and if I can't tighten it beyond about the 80% depth, replace
with a screw of smaller diameter.