tom, venting into the attic is a bad idea and against most (probably all) model building codes. If you don't want to go through your ceiling, maybe a through-the-wall model would work for you. I believe the minimum volume called for by the UBC is 5 air changes per hour, but 8 is more commonly used. The volume of your bathroom is 600 cu. ft., so (600 x 8)/60 = 80 cfm. I think Panasonic makes a 90 cfm. (check www.tamtech.com). You should be aware that bath fans don't have a lot of power behind them, so you need to keep ducting as short as possible, and make all bends as gradual as possible. If your duct path gets nasty, you will have to go to a larger model to get the cfm you need.