OMG.....Duhh.....I found the article where I got this equation from.....
Its at the Krib, under plant lighting FAQ

here is the link to the page:
FAQ: Plant Lighting

check it out........
can I use this to determine how high to hang my lights, regardless of the total watts of the fixture?

on a side note: is it possible to use 36w CF tubes in the 65watt square pin sockets? I know that it is ok to put the 55w bulbs in them, but is 36w too low?

switching to 2 36 watt tubes would allow me to hang my fixture MUCH lower.