As a recovering IV meth user I really encourage you to not shoot meth. You might get a couple of good shots and then meth is basically ruined forever for you and all you get at best is an addiction and wrecked veins.
But if you insist, I’d start with a quarter point at most. However, remember vasoconstriction makes getting a second shot very difficult. Don’t bother trying. If you are not high enough this session just smoke or snort some more and try a bigger shot next time.