For your bathtub I would suggest that you use a 18″ grab bar at the entry point of the tub, hung vertically. In addition, use a 24″, 30″ or 36″ grab bar hung horizontally on the back wall. I also suggest that you have your towel bars replaced with ADA Towel Grab Bars or a moen peened grab bar. Using towel grab bars in all of your bathrooms will reduce the chance of injury for that first slippery step.