There was nothing vaguely vague about any of Dr. Collett's rules of thumb. There are two constants in the equation usually applied to film and developer, which is presented along with a simple form of the Arrhenius equation. They allow for different slope and intercept on semilog paper graphs. Over any reasonable range of temperatures I would allow in my darkroom, the simple equation works very well. If you have only one experimental point, you can plot another one at 10 C higher and half as long, or 10 C lower and twice as long and draw the line between those points to get a very good guess at your next experiment.