Sub cable length shouldn't be an issue - get what you need to put the sub where you need it, and be sure to avoid running it in parallel with the power cord (close proximity to 120VAC could yield a "hum" from the sub).

Keep in mind also that the standard Radio Shack SPL meter is not perfectly accurate at low frequencies. There's a correction chart out there somewhere to compensate for that, and if you haven't done so already you may be running hotter than you think. It's just something to think about. Once you get the sub into its final place, you might want to experiment some more with the trim settings.

Glad to hear that the new crossover settings are working well for you!