John:
There is, in fact, a more complex formula for determining the stimp of a green based on the two measurements ... it assumes a consistent slope (whether it's 1% or 4%) and corrects for it. I've seen it in the USGA Green Section publication and also somewhere else.
In the original Green Section Bulletin article about it, there was also a little chart used to show what the Stimpmeter reading on a downhill putt of 1%, 2%, 3%, 4%, or 5% with greens stimping 8, 9, or 10. This chart was quite fascinating. At 1% it only makes a little bit of difference, but once you get up to 3%, the "effective" green speed of a green stimping at 10 was something like 27 ... and at 4% the "effective speed" was 81!!