I am installing a American Standard Princeton bathtub. I removed underlayment floor in the bathroom down to the subfloor. Unfortunately the subfloor is only 1/2" thick. Also the floor joists run the same direction (length wise) as the tub. I plan to install a 3/4 plywood underlayment. Since the subfloor is only 1/2" would it be wise to install the 3/4" underlayment under the tub first for better support instead of installing the tub on the 1/2" subfloor as the instructions suggest? Would there be any problem with this or is it a good idea?