Since the machine shop likely used a milling machine to 'flatten' your head, I'd get a sheet of glass and some 220 wet-dry and sand it flat. Milling the head is often not accurate enough. I removed one head from a tractor that had been 'flattened' with a milling machine, and it was more than 0.010" out of flat
When the head is placed on a sheet of plate glass, a 0.003" feeler gauge should not fit anywhere between the gasket surface and the glass. Any more of a gap and it isn't flat enough. Mine wind up flat to within 0.001". Due to the different expansion rates of the iron block and aluminum head, you need all the help you can get.