Mike King wrote:

Kodak's Darkroom Data Guide once had a dial used to figure time and temp for
all their films, by extrapolation I included films by other makers but ran
into problems when the TMax films came out.

The dial works (in my not very well tested experience) for time
versus temperature, while film and developer are held constant.
That is, if you know the right time at 68 F, you can predict a
reasonable time at 72 F. But you already need to know a right
time for your film+developer combination. For Tmax, I used the
film box or datasheet value as a starting point. The dial is basically
just a convenient version of a time/temp chart like this:
http://www.digitaltruth.com/chart/timetemp.html

Dan wants a formula that lets him convert times from one developer
to another, or one film to another. I don't think this is possible
with a simple conversion. If this were possible, the time for film X
would always be a constant factor times that for film Y, and
some study of a dev chart will show exceptions.

For example, a very small amount of data from the
Massive Dev Chart:

35mm,20C HP5+(400) FP4+(125) Tri-X(400)
Acufine 4.5 4 -
D-76 7.5 8.5 7-8
D-76 1:1 13 11 10-11
HC-110 B 5 9 4.5-6
Rodinal 1:50 11 15 13

Now, maybe these times are all crap, although I think some
of them are from manufacturer's datasheets.
But my point is you can't come up with blanket statements
like FP4 takes 50% as long as HP5, or HC-110B takes
2/3rds as long as D-76. And I deliberately picked three
films that are fairly similar, so the difference isn't
regular vs T-grain or anything like that.