Here's a trick.. if you're using photoshop to color it.. erase part of the forearm and use the rectangular marquee to select the upper part where the hand and all is and 'cut' it. Then make a new layer and paste it into the layer. Use the move tool and move it into position where you erased a part. It'll shorten it and all you'll have to do is touch up a bit and reshape some. It's a quick way without erasing everything and having to start the arm over.
OF course you might already know that, or know a better way, but I tried.