I carry an Emerson commander, folding pocket knife with about a three inch blade. The blade is wider at the outward curve on top and the center part of the knife has sort of a concave curve to it although not as exagerated as the top curve. I tried sharpening it as I would a straight blade with disappointing results. Any tips on how I would go about sharpening a blade like this and maintain an even edge angle throughout the blade?