Screw length & thread engagement
Too short and it pulls out. Too long and it bottoms out or punches through. This gives you the minimum bite and a length to buy.
Screw size
Threads into
Minimum bite
3.0 mm of thread
Total screw length needed: about 5 mm (round up to the next size you can buy, then check it does not bottom out).
One diameter of thread is plenty.
Two things that catch people out
- A blind hole is often deeper than the threaded part of it. Test with the screw dry first.
- In a countersunk hole the head sinks in, so the thickness you type here should be the part below the cone, not the whole panel.