This is how I determine limb thickness on selfbows. I use my hand and a pencil as a gauge to mark down the entire limb from tip to tip and on both sides. This will help keep your limb thickness even from side to side. After I have found the back ring and cleaned it from one end to the other I will put a pencil mark at about 3/4" to start with and usually remove the excess wood with a band saw starting on the edges with a beveled cut down both sides then reduce the crown down the middle...then check floor tiller.
Next I go a bit closer to the back(1/2" to 5/8") with the same method but begin reducing to that line with a farriers rasp using the same method or removing the corners first then the crown...and check the floor tiller. I continue this until it floor tillers to about 4" to 6".
Now it is time to long string and check both limbs together. From this point on it is all done by the bend of the limbs.
You will notice that the pencil line mimics the back of the bow following any undulations