I used to use a target thingo, found it a bit fiddly, now I use a simple old sewing tape measure. I've got my lens focal lengths marked along one edge, and D-stops (in inches) along the other edge. Quick and simple to hold it up and see how many stops between the focal length and actual bellows draw. I never used to look forward to calculating bellows factor 'till this little strap came along, can't recommend it enough.
I do a similar thing with a length of string with a knot every two inches. It's quick and easy.