I do it by magnification. If you hunt around, I've posted a table that converts magnification to exposure factor as a DOC file, which you can tape to the back of your camera or to your light meter as a handy reference. It works for any format, and you don't need to know the focal length of the lens or how much extension you are using.

With MF or 35mm, you can estimate magnification by comparing the size of the subject to the size of the image on the groundglass, since you know the dimensions of the groundglass, or you could put a ruler in the scene at the subject position to measure it more accurately as needed.