+Daniel - I don't know if I understand your question completely. You mean you've completed a round at a certain course, and then you edit/update the rating and slope subsequently, and are looking for the index differential to change?
I get this message due to a 9-hole round, though this doesn't seem to inhibit the calculation. Would that be your case as well?
Note it doesn't say how this can be fixed on that screen. Also, I find that the handicap will not update from a recent round unless I go into Settings/User Profile/Edit contact and photo to get to the handicap index. Then it updates to other portions of the app. (Of course, I do know that the handicap doesn't necessarily change after each round depending on which round 'drops-out' of the standard calculation).
Our calculated handicap uses the USGA formula so it will produce the same number. But if you wish to use a handicap for a tournament that must come from a licensed club. We are not one of these clubs at the moment but may look into partnering with one in the future.