I assume you're using 11 or 17 inch tape since you're asking how long to make it? 9 inches should be fine, but you'll want to cut more to account for the clips and overhang of the tub. It will probably end up being like 11 inches. Just put the tape down where it's going to sit, then put the tub down where it will sit. Make sure it is sitting on 1/3-1/4 of the tape and then cut it.