Calculating the length of tube for DIY Nitrate reactor?

[BCOrchidGuy]

Okay so I'm intruiged, I looked at them a couple years ago and it seems they are still popular so I'm guessing they are working for people. I have a question though, I've seen lengths of anywhere from 75 feet of line to well over that. Does anyone know of a link etc that would tell me if there is an actual calculation to determine the run of tubing I'd need or is it just by guess and by golly? I can buy a 500 foot roll of airline tubing for a song but I'm assuming that would be way to much, or would it? Would I end up with a killer Denitrator or would it just be a waste of tubing?

Doug