How to count the units moved with 2 motor encoders
im not sure of my answer:
arc length = $$2\pi r\theta$$

distance between wheels = h
arc1 units = $$2\pi r\theta$$
arc2 units = $$2\pi (r+h)\theta$$
arc_center = $$2\pi (r+(h/2))\theta$$
average:
$$(arc1 + arc2)  / 2 = (\pi\theta)(r+(r+h))$$
$$(arc1 + arc2)  / 2 = (\pi\theta)(2r+h))$$
$$(arc1 + arc2)  / 2 = (\pi\theta)(2(r+h/2)))$$
$$(arc1 + arc2)  / 2 = 2(\pi\theta)((r+h/2)))$$
you can see your method is actually = center arc length