If I know that someone is going to use the Phoenix "High RPM" mode, I can set up a single pot to cover about a 2,000 RPM range (say, 9,000 to 11,000), as in the FM-2 and FM-2R, with throttle values between 20% and 50% of full throttle (1.20 ms to 1.50 ms pulse widths). But other modes, such as the "Simple RPM" mode and the "Low RPM" mode have a different calibration, as do the Hacker being sold through RSM and the Schulze F2B. With two RPM pots, as in the FM-2a and FM-6a, it is possible to cover the whole throttle range and so it can be used with any governed RPM mode whatsoever; the idea is to center the "fine" pot and then use the "RPM" pot to get the middle of the RPM range that you want, and use the fine pot thereafter for small changes. In each case, it is possible to make changes in throttle values of about 1/2 of 1%, probably as good as the ESCs can utilize. All of these timers allow the user to adjust and tach the RPM in the first minute of "flight" time. And, in each case, the warning of the end of flight time is made by selecting one second worth of lower RPM.
The "Set RPM" mode of the Phoenix is quite different in its requirements; you get RPM #1 for throttle settings below 50%, RPM #2 for throttle settings above 50% but less than 100%, and RPM #3 for 100% throttle. I can set up a single RPM pot to easily access these three RPMs; for anything close to fully counterclockwise, a single pulse width in the less than 50% throttle range is sent; for anything in the middle of the pot's range, another single pulse width in the 75% range is sent; for anything close to fully clockwise, a full throttle pulse width of 2.00 ms is sent.
A timer with two RPM pots could theoretically access all three RPMs, but the warning RPM would always be RPM #1, thereby making that RPM rather unusable. So I normally set up a single RPM pot timer for either High RPM (or another simple governed mode such as the Schluze) or the Set RPM mode. In the Set RPM mode, the warning is given using RPM #1 if that is not in use and RPM #3 if it is--placing a requirement on the user to select RPMs that differ in 500 RPM or so if the warning is to be noticeable (I tried initially to use a momentary "off" throttle, but that takes far too long to recover.).
The FM-6a is unique in that I used a larger version of the microcontroller and so could incorporate both two RPM pots and a switch to select either the program for an adjustable, governed RPM for any ESC or the program for the "Set RPM" mode of the Phoenix. But since there were two RPM pots and the "Set RPM" mode really only needs one, I added a couple of optional variations for using the three RPMs via the "fine" pot.