Basically, one of the constants shows till which level the hero can get exp from creeps (standard is five). The following ones represent the percentage of exp the hero gets from the creep kill. Level Factor is just for the specific level, while Constant Factor sets the percentage for every level (dunno which of both has higher priority if both are filled, but I guess the Level Factor). From here, you should be able to figure out how it works. Just trial and error.