One way I have used in the past on longbows.
Work out the thickness every 6" or other convenient distance.
Rasp/file etc the belly to give that thickness for just the width of the rasp at eack of your 6" marks, then use spokeshave/rasp light sabre, whatever, to blend in between the spots. Use fingers to check for even blend.
It's surprisingly quick and effective.
BTW, I find it very good to use inches for measuring along the bow but mm for thickness as they are a nice convenient size and often the nearest 1/2 mm is close enough.
E.g On a longbow, I rough out the taper to either 1.6 or 2mm every 6 "
Del