I've trained a spreadsheet to calculate room cavity ratios. Using a 10' ceiling height and an assumed workplane of 30" above finish floor and the other room dimensions you listed, the RCR came out at a little under 2.
Then, I pulled out a really old lighting design book (Rapid Lighting Design and Cost Estimating by Prafulla C. Sorcar, P.E. copyright 1979), looked up the 2x4 fixture curves, assumed 30 fc & read the curve over to the square footage per fixture, divided that into your square footage which worked out to 19.xxx fixtures.
As for the 18fc estimate, I did the same thing backwards by dividing the square footage by # of fixtures, read over to the curve & up to the fc.
I'm not sure how accurate this book is, but I've never had any complaints about things being too dim.
BTW, Prafulla has published other lighting books & is multi-talented, visit his site at www.manicksorcar.com
[This message has been edited by Tom (edited 07-12-2004).]