It's common for wooden bows to develop more set in the lower limb, since that limb takes more stress while being drawn in the hand (as opposed to most tillering tree setups). The lower the heel of the hand holding the bow drops, the more the center of balance shifts, causing the lower limb to have to bend more at full draw. Most of us will leave the bottom limb stronger (creating a "positive tiller") to counteract this. I always advice a high-heel grip, very similar to that of Fred Asbell. This creates a more symmetrical fulcrum point and stress load between the limbs. (Interestingly, some traditional archers/bowyers actually flip their bow over when this problem develops, thus making the top limb the bottom and visa versa. It isn't always feasible, but sometimes is a quick and effective solution.)
That said, the advice given is good...slow wood removal over the length of the upper limb. If using a scraper, I find it best to pull it along the belly at a 45 degree angle to avoid creating, and eventually deepening, chatter marks. Switch the angle as you go. Also, the more you hold the scraper blade a 90 degrees perpendicular to the bow, the more prone to chatter marks you'll be. These are not only unsightly, but problematic over time.
Sure hope that helps!
