Here is what I do;
I mark a line every 6" down the limb on both sides of the limb. I start at the end of the fade and make my first measurement, because I make slightly round belly bows I start at 1/2", every 6" I drop my measurement 1/16th" until I get to 1/4" at which point I carry this measurement to the tip of the limb.
With this the 1/2" measurement all the way to the 1/4" measurement I am rasping to a peak on the middle of the belly so my actual limb thickness is more than 1/2" even at the tip.
It is easier to get a uniform line doing it 6" at a time. I call this line the roadmap for your bow, don't violate it. If you have too much poundage after tillering the belly almost flat drop your side lines another 1/16" again stopping at 1/4".
I use the finger nail on the back of the bow to guide my pencil, you can get good at it with practice, the eraser is your friend. Sanding the sides of your limb and keeping them square makes the pencil marking easier.