Works like a charm! Changed to ICP1, digital pin 83, but otherwise ran it as is. Now I will have to figure out how you did it! 
Thank you very much!
[BTW, both of Nick Gammon's sketches work fine on the 2560, but I simply wanted to get both frequency and duty cycle in the same sketch. My hardware is already made, so I am stuck with the ICP1 Digital pin 83 for input, and "they say" this pin is not one of the pins available for interrupts. ("Mega2560 DIGITAL PINS USABLE FOR INTERRUPTS 2, 3, 18, 19, 20, 21") I made a PCB using the atmega2560 chip, so I have access to ICP1. There is also a lot of other processing going on at the same time, so I was trying to minimize load on the system by using input capture.]