Measure from the point in the riser where the taper fades into the limb straight horizontal to the string. The top limb distance is typically 1/8 to about 3/8 inch longer. That is normal split finger tiller.
Correct, and to add to that - the low point on the grip where your hand contacts the bow is generally the physical center of the bow (on a recurve, but not necessarily true on a longbow). Where your fingers contact the string to draw the bow is not the center (mid-point) of the string. This requires one limb to bend more than the other and generally the upper limb is made slightly weaker accordingly. The only thing that matters is that when you release the string, ideally both limbs should recover such that they arrive back to the undrawn position at exactly the same time. As R H Clark mentioned, tiller is typically measured from the string to the belly side of the limb at the fadeout which is where the riser fades into the limb. For my bows and me shooting split finger, 3/16" positive tiller is about right, meaning the dimension from string to limb is 3/16" greater on the upper limb than the lower limb at the fadeout. The actual tiller measurement is not as critical as the limbs recovering to a stop at the same time.
As mentioned, if the bow shoots good for you then you are good to go and nothing else matters!