Originally posted by asharpe 'course if you want accuracy, get an extinction meter.
I have one, and it actually isn't bad! A little square thing, with a pop up hood. I can't find it now, unfortunately, but it's an expophot; I'm sure you've seen them. They worked on Sunny 16.
My favorite meter is the GE PR-1, you can find them on the auction sites for $5, and out of the three I bought, the first one works beautifully. It does incident and reflected readings if you get the incident cover. The ISO can be set as low as 2, which makes it valuable for my paper negatives. It's practically indestructible.
I use it mainly to calibrate my sunny 16 settings.
I take a reflected reading of an average scene, to set a baseline, then evaluate the actual scene to choose the settings I want to use.
My next favorite is the GE DW-68. Which is also nearly indestructible, has about a 1 in 3 success rate from the auction site, however it will only do incident light in very dim light, closer to EV5 or less.
Never used an extinction meter.