0.5 mm should be good. It's a lot easier to grind a little off a little more than grind a little more back on.

Too tight!unless you can just lift the hatch away all together.
If ,what usually happens,the hatch is prised open from one side then it rolls out of it's hole.This then causes very high pinching forces on the vulnerable edges of the hatch left behind.This then chips and damages them and then water gets in.Don't ask how I know this!
I now use about 3 mm of a gap between the hatch and it's surrounding deck hole.This falls within my poor jig saw cutting skills nicely.
cheers,