Telescope Reviews: CPC1100 Focal Length: Range?

I was "calibrating" my Celestron 17mm retical eyepiece, which has a 0-60 scale right across the center. The extensive instructions say that in order to calibrate this scale to my particular telescope, I should pick a star and time it while it drifts across the length of this scale. Then plug that time (54 seconds) into an equation to get the exact number of arcseconds per division. Trig doesn't scare me, so off I went, using Arcturus. (Declination -11 degrees.)

The first equation is for those who happen to know the exact FL of their scope, while the second equation is for those who do not, or who own a SCT, which has a variable FL that depends on the actual location of the primary, which moves when you turn the focus knob. They go further, explaining that SCT manufacturers publish FL averages, not the exact value. Okay; I buy that.

My CPC1100 (with the reducer/corrector) is f/6.3, with a published FL = 1764.

When I use the proper equation I can solve for FL (to check my work) but I get 1500+ which is no where near 1764. I checked and double checked, but I think I did it right.

Can this 1500+ FL result be within the "range" of a CPC1100 at f/6.3?

Thanks in advance.

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