Hmmm....your pool is the same size as mine. I see the specs on the Stenner 45MHP2 is .2 - 3 gpd. I assume for your 13,500 gallon pool that you must leave your pump set at the minimum .2 gpd setting most of the time. That would be adding 1.7 ppm FAC per day, or 1 gallon every 5 days. Just curious where you get your bleach and what you pay. We have a company here in Buffalo that manufactures Chek-Bac Chlorine. It is sold in pool stores and in my local grocery store. Needless to say, I buy it from the grocery store for 3.99 per gallon. According to the bottle, it's labeled as 12.5%, but I do all my calculations as a straight 12%.