Homebrew PCBs

When buying an HF ballast and fluorescent tube does the ballast wattage have to match the tube wattage?

Is it allowable to use an hf ballast of higher wattage that that of the tube  e.g. hf ballast 40W tube 12W ?

Can you parallel tubes to an hf ballast intended for a single tube so long as ballast wattage is sufficient ?

I am building a uv-a  light box for sensitised PCB exposure and am not sure how many tubes to use.
I am thinking get a higher wattage than needed hf ballast and say two 8 or 12W tubes so if there is insufficient light I could get one or two more tubes and parallel them.

I usually use the sun for exposure but there has not been much of it here over the last few weeks and I need to make a PCB.

Thanks 
John G3UGY