Since you're batch sparging, it doesn't matter. The gravity will only change during the lauter if you're still converting. What I do is check it at the beginning of vorlaufing so that I know whether I need to extend the mash rest.
Did you check gravity for the second (and higher, if any) runnings? To get 76% overall efficiency with 79% conversion for the first runnings you would almost have to have gotten additional conversion during the sparge(s).