Soon-to-be owners of a 28' AGP. Buying a gas heater because we want to extend the swimming season from 5/1 - 9/31 in Rochester, NY (average temperature in May is 57 degrees). The salesman who sold us the pool insists 125 BTU gas heater is sufficient. He says online calculators are for IGPs which require more BTUs to heat. I'm not convinced. Is he correct? A higher BTU heater isn't that much more expensive and I will kick myself (or him) if that's not right. Thanks!