Usually one measures the distance from the string to the riser at points equal distance from the center, generally at the "fades" or limb attachment point. Generally the upper limb distance will be 1/8" to 1/4" wider. Bows tillered for three under use less difference, 1/8" to even. One can usually adjust the nock point for his unique shooting style. If you notice a dramatic difference w/ 3 under, you might consider moving your nock point, up a bit at first but experiment.
Many will argue, fairly at least with glass bows, that tiller "doesn't matter." But I think antecdotes about a bow with some positive tiller being "louder" when shot 3 under have some merit, at least in my experience. With selfbows a significant change in where one applies pressure to either the bow or string, relative to how the bow was built, frequently can result in more fatigue on either the upper or lower limb.