TL;DR: Use the ruleset you chose when creating your campaign to populate a campaign-level "Default diagonal" setting, which can be over-ridden. When creating a map, this default is used to set the diagonal distance, when can again be over-ridden on a per-map basis. I find it annoying to change the diagonals distance every time a create a new map, and with the current default using the 4E "people move 30% faster when traveling in non-cardinal directions" system that no other ruleset I've played uses, this is a constant annoyance. I was going to suggest just changing the default, but that might annoy 4E players, and other rule systems might well exist that uses the 4E diagonals that I don't know about. The solution seems to be to just query some info that Roll20 already knows (What game system you are running) and set the default accordingly. Ideally, the default could be over-ridden in the campaign settings, for those that don't like the official diagonal rules of the system they are running. Yes, some work-arounds exist (Creating a blank map, setting the diagonal, and then copying that map to create all other maps comes to mind) but they are a pain, and I think using the existing info to set a default is a far more elegant solution.
