This is a simple formula I've been working with to reduce temp of stock to by adding cold water to make a working solution to be used immedieatly.
A = DESIRED FINAL TEMP -- F
B = TOTAL # PARTS OF BOTH SOLUTIONS
C = EXISTING STOCK TEMPERATURE -- F
D = # PARTS OF DILUTION WATER ONLY
X = NEEDED TEMPERATURE OF DILUTING WATER ONLY
To make an 8 oz 68 degree solution from 72 degree stock solution at 1 + 2 ratio would be:
This works for me when tempering small amounts for processing rolls of 35mm. It may not be sophisticated, or beautifully written, but it works.