Once you remove the bark and the sapwood,you then can generally see the grain good enough to lay out a little fatter than needed front profile,and then reduce your width then chase the ring. That valley will get shallower when you narrow it.
Oh...and im not sure what's a safe enough depth to leave alone,but if you put a straight edge across the valley(after you narrow it to bow profile) and measure down and if its deeper than an 1/8", then id fill it in with sinew until the sinew is higher than the edges.
I've left 1/16" valleys on a couple of mine and there fine. Its not too uncommon to encounter this with osage.