Looks like I answered my own question here:

For example, in a bathroom, subtract the tub, vanity and toilet areas. In the kitchen, subtract the cabinets, refrigerator, oven, etc. If not heating the whole floor, plan to heat at least the main traffic and living areas where people will be walking, standing or sitting. The heat will only spread 1 to 1.5 inches from the radiant floor heating mat. So, the surface of the unheated floor will be noticeably cold compared to the warm floor. Heating wires spaced more than 3" apart will leave cold spots. On the other hand, wires should not be spaced closer than specificed in the installation manual .