Hi Bill,
Your definition of dutycycle differs from mine but I think I understand what you want.
The period (on-time + off-time) of the signal is 18-200ms and the on-time is 500-1000us. You want to measure the dutycycle, ie the ratio between on-time and period.

If the period is 125ms and the on-time is 565us the dutycycle is 4.52%.
If the period is 45ms and the on-time is 795us the dutycycle is 17.66%.

Is this the number you're after?

Let's see if we can make this easy....
The longest time you want to measure is 200ms. Using a 16bit timer with a 4us "tick" you'll measure up to 262ms with 4us resolution without having to worry about overflows. Then the overflow can be used to signal invalid mesasurment (motor not running).

Are we on the right track?

/Henrik.