[Jeff Lee]

You can reduce the runner volumes by cutting the intake or exhaust surfaces as mentioned. Rule says you can not go past the valve cover bolt hoe.
You can also reduce those volumes by sinking the valve(s) deeper in the pocket. With some valve jobs you can gain flow by doing this correctly. Key word is correctly.
If it was a particularly rare head I would look at all avenues. If it's your dime-a-dozen head I'd scrape it.
And no, there is no tolerance on the plus side.