What Don said, or, I usually start the taper an inch or so in front of the stab L/E and then only bring it down to 1/4" or 3/16" depending on the rest of the tail design.
IMO, this is one of those things that doesn't need to be overthought a whole lot. Forget the plans and go with your gut. It's definitely better to leave a little extra "meat" than take too much. Keep it strong in this area of the build more than worrying about weight. Mainly just take into consideration the hardness, density, stiffness etc. of the wood itself.
Or you could do this, sand in the taper afterward, and then sheet it...