Not being much of a scope person, I don't understand a lot about scope ring height.  

Most manufacturers list at least a low, medium or high, but is there an industry standard numberwise as these heights are measured in thousandths of an inch, if I remember?  And from what point on the ring is this measurement taken?

What ring height generally is the best for co-witness of red dot sights with the front sight on ar15s (RRA TAC handle or flattops) for 1" sights and 30mm?  Is it different if you use the 30mm rings or the 1" rings, which some red dot tubes are?