I have slept on a waterbed since the 70's. Last year I had to replace the mattress. The waterbed mattress measures 80 inches by 84 inches by 10 inches tall. I filled the matress to capacity with water that I thought was about 98 degrees, but after the bed was filled I found out that the actual temperature was only 78 degrees. It took 3 days for the waterbed heater to warm it up to 98 degrees. I had to sleep on the couch for 3 nights. One thing that I could have done was drain out part of the water and replace it with 125 degree water. If I had done this, how many gallons of the "colder" water would I have had to drain out of the bed and replace with the 125 degree water, so that the temperature of the bed would have been 98 degrees?