Rather than doing the math, an easy rule of thumb is for every 50% extension over the actual focal length you need to double your exposure (twice as long a shutter speed or open 1 stop).
I've been trying to figure this out for a while and I eventually bought a leaflet of ebay with macro tables for d.o.f etc. Basically you open up 1/3 of a stop. Unless you extension is at the same length of the lens or over in which case its 2/3 of a stop. I'll try and scan the leaflet for you, its a photocopy though so quality might not be great. Does specify what d.o.f is with certain extensions and lens types, switar etc.
Good Luck!