I think the point is that in the time the engines are spooling up the acceleration rate is less than had they been at TO thrust from the start, so the ground roll to that point( the point where TO thrust is set and the two cases reach the same speed) MAY be longer or shorter depending on the entry speed for the rolling example. Unless you are prepared to give minimum threshold crossing speeds I'm not sure how you could realistically predict the initial ground roll distance up to the point where TO thrust is set and therefore calculate the remaining acceleration.
The Aft limit of the CG envelope can be limited by brake release tip up to ensure no probs with setting thrust but I agree this doesn't apply to a tail dragger.