I posted this in the bellows extension sticky. I measure from groundglass to lensboard and do the following:

I know this has been well and thoroughly covered but perhaps the way I prefer to do it will be of use to somebody. I don't really care for doing math in the field. I want everything prepared for me. I just sit down at my computer and fogure out the bellows extensions for a given focal length that will evenly correspond to stop corrections in 1/3 stop increments going out as far as the bellows on my camera will allow. I then make a small table with P-Touch tape and stick it to the lensboard for that lens. That ensures that I can simply set up the photograph I want and then choose the closest stop correction from the table.

Here's how the tables look, this one is for a 90mm:

90mm

+1/3 - 106mm (4 3/16")

+2/3 - 121mm (4 3/4")

+1 - 127mm (5")

+1 1/3 - 142mm (5 9/16")

+1 2/3 - 156mm (6 1/8")

+2 - 180mm (7 1/16")

+2 1/3 - 202mm (7 15/16")

+2 2/3 - 226mm (8 7/8")

+3 - 255mm (10")

+3 1/3 - 285mm (11 1/4")

+3 2/3 - 321mm (12 5/8")

+4 - 360mm (14 3/16")

+4 1/3 - 404mm (15 15/16")

+4 2/3 - 454mm (17 7/8")