Setting air/fuel screw on 2 stroke dirt bike?

I have an IT 200 that I'm rebuilding the top end on because of a piston meltdown. When I'm finished how do I know the mixture screw is right so this doesn't happen again. I have Clymer manual but it is very vague on this. Thanks.
deck: it does have a mixture screw "and" an idle screw.
 
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