[gofastnut]

The way I've done it, twice only, was to lay an intake gasket (that matches the intake manifold) up against the head and use a black marker to trace out the shape of where you will want to remove material to blend in, and be all in by maybe an inch.
I know this is called gasket matching, but if you can get a junk cylinder head and practice this part, you'll get an idea of what needs to be done to the rest of the head.
Always take too little, it's way better than taking too much, which you can't put back!