Well, if I wanted to install a tub drain and do it the most difficult way, I would follow your plan. If I had to remove and reinstall the tub more than two times, I would consider myself a failure. You should do it one of two ways
1. Install the drain assembly on the the tub, THEN measure and position the trap after measuring where it should go. Next, remove it and then fasten it in place at the proper location and height relative to the floor as determined by the tub. finally set the tub in place over the drain and make the final connections. OR,
2. Measure where the tub drain and overflow openings are and then construct the drain assembly do that the fittings are in the correct locations and then set the tub in place. This option is usually only necessary when the trap is not in the right location and the drain assembly has to be offset to make it work.