I just use a tape measure.
If I'm using an 8 inch lens (203 Ektar) then at infinity I measure from a point on each standard that are 8 inches apart.
If I focus on something and the standards are 11 inches apart, measured from the same points, then I know to allow one stop extra exposure.
At a distance of 16 inches I know to allow two stops extra exposure.
You can also work in fractions of a stop i.e 9.5 inches or 12.5 inches.
This also works with my other lenses 101 Ektar = 4 inches, 135 Optar = 5.5 inches etc.
I don't know if this is accurate enough for colour transparency work but it does well enough for black and white.