How much would a flat square be curved, on Earth, if is was 512x512 miles? Would you even be able to notice? Besides, this is a WW2 game, and we don't have weapons that can reach out far enough for it to make any difference. All the speed calculations are done for you, and as pointed out, they are pretty dang accurate. Would having any more precision by knowing the precise terrain curvature give you opportunities in the game that you don't already have?