the differences come in where you might be retrofitting a 4 stroke with a fairly long shaft , or sticking a short shaftengine into a spot where you had a motor with a shaft extension. If ou just pick a fixed point, like the edge of the backplate, the calculation could be off enough that you might as well just cut and try. That is what I usually do anyway. Good planes don't seem to be terribly critical of an ounce or so of nose weight, most of the time.