My daughter is having a birthday party on March 2nd and wants to heat the pool. I am trying to figure out how long I will need to heat it to get to temperature. I am thinking a water temp of 85 should be warm enough for kids. I am located in Houston, TX so there is no telling what the weather will be like in early March. Could be 90 outside or 40. Currently our water temp is 61 degrees. Pool is roughly 18,500 gallons and my heater is a Jandy LT400N-L and is rated at 399,000 btu / hr. Is there a formula for me to determine how long it will take to heat my water?