As prop swinger says, you would probably take the standard hours to pass your PPL anyway even with the hours reduction available. I had 20 odd years of gliding under my belt when I did my PPL but it still took me 45 hours with the skills test. Mind you, about five of that was staying on the ball while I could get a skills test sorted with the right day/weather combination.
IMO you can never waste time or money gliding. I know others disagree but I'm certain I'm a better power pilot for it. I always use the example of motorbykes and cars. I had many years on bykes before my first car and I think it makes you a better driver.