Well, this is embarrassing. I made RTM_TimerCalc such that it applied PreScale to the External Clock. Unfortunaterly, the Arduino timer's don't work that way. So this is way off.
Be advised, you can't use the External Clock facility for anything other than calculating what a "different" (other than 16 or 8 MHz) clock hooked to your Arduino would do given certain parameters. In other words, this feature applies only if you want to change the 16 or 8 MHz crystal to some other frequency.
That said, it seems a shame to remove the current facility intended to help with external calculations. So here is how things will play out for the upcoming Version 1.20.
- The External Clock designation will be changed to "Custom Clock". The current capability will be unchanged (it -will- be Prescaled).
- A later Version (after V1.20) will include the capability for a non-Prescaled External Clock and its associated calculations (and error messages).