Bruce, I'd need a spreadsheet for that!
I have the same problem in summer and use the diluting water to temper my developer. I make up a bucket of wash water at the required temp and place my pre-mixed bottle of fixer in a jug containing water from the fridge. While I'm agitating the developer, I agitate the fixer (shake the bottle a bit) and check it's temp. I usually have to remove it from the cold water bath before my development time is up as it's down to the desired temp.