I've been thinking about this for awhile, grades are a rough estimate but very arbitrary, so I've been giving personal ratings to things I do outside. Generally, I down grade things but not always.
I've come up with a simple formula that seems to work for me I'd like to share for discussion and to be flamed.
It only works starting at V4 IMO.
You break a boulder problem down into two sections, rate each of those two sections, then add 4 and divide by two.
The two sections can be just one move thats say v5 or it can be two sections that are a comination of moves.
So if you do v4 into v4 it goes like this, 4 + 4 + 4 / 2= v6
v10 into v8, 10 + 8+ 4 / 2= v11
v6 into v8, 8 + 6 + 4/2= v9
Anyways, I was surprised that I found it to be very accurate when I applied it to boulder problems I've already done, almost perfect.
v14 into v14, 14 plus 14 plus 4 /2=v16