Here is a solution I came up with that parallels the circle spot for straight fingers pointing forward. I propose a square spot or box for curves going forward for the flat hand. The square is to show the nail of the hand. If the curves go left or right, they are drawn that way, but if the curves actually bend forward or back you'd show the nail. 

That way it is a true position, not a compromise position that can be confused.

I tried to create this in powerpoint but lost the picture.