The size, type, and distance will all play a roll in your exposures. You will have to do some testing and what you propose sounds like a good way to start.

It might be a bit more quantifiable if you had a step wedge, but Cyanos are not terribly expensive so a few tests shouldn't put a damper in your wallet.

On the max print size, you will simply have to test and see what light fall off you have with your unit. The light falls off as a square of the distance so print times will increase quickly, so yes, it does apply.

On your last question, as long as the tubes can match the ballast, which sounds to be the case, you can use black light bulbs. Many Aquarium bulbs are in fact, UV intense, as you say.

If you really want to shorten times, find a HID grow light and ballast. Here in the states I've seen one for about $180 which would do a bang up job.

Best of luck