Each developer I think would have a different rate of adjustment so I doubt there is a formula that exactly meets your needs. But what you might want to look at is the temps that wanders off the range. Temps effect the reducing agents at different rates so that unless you've got a specific reason for straying too far from 68deg, it's worth while to try to standardize. I don't use tap water so my bottles of distilled water normally pour out at exactly 70degs and that's my standard. I hope this helps.