I gave up measuring and calculating years ago because it's too much work. For $7.99 you can buy the Calumet Exposure Calculator from Calumet Photographic (www.calumetphoto.com) - it's catalog item number CC9201.

You place the measuring square at the subject position and measure on the ground glass with the exposure compensation ruler that is part of the kit. You get a direct reading as to how much more exposure is required.