I am installing a Kohler K-9135 shower drain in what will be a mud job not a shower pan.My question is should I cut the drain pipe now so that the bottom flange of the unit is flush with the sub floor,or leave the pipe long and cut it when the mortar is installed?.It seems to me common sense tells me to cut it flush with the sub floor,but I really am not sure how to go about it. Thank You CoryC