A Calc button has been added to the match details screen to help assist with win margin calculation:

The button enables when a win is selected as the match result. When pressed, it will do a simple margin calculation based on the winning team/innings scores and then populate the appropriate margin text:

Limitations - it isn't quite smart enough to understand a win by DL or where a team is forced to follow-on so these still need to be entered manually

Update: It will now be enabled for 2 innings games where the win is by outright. "1st innings" wins are a bit more complex so the button will be disabled for these.