I use a very handy plastic laminated FotoSharp Guide, "Macro and Closeup Exposure Adjustments". It covers focal lengths from 75mm to 300, subject magnification from 1/8th to 1:1 to 3:1, distance(s) from subject in inches, and gives adjustments in Fstops or time changes. Fits in my back pocket. These are available on Ebay all the time. Or direct from Bob at 800 361-8341. I don't know Bob, and have no interest in his enterprise...hope it is ok to post this

I do a lot of close work, and this has never failed me, yet. It saves me one more right brain to left brain shift. I would much rather concentrate on the delicate drawing of light on a petal, than start doing squares and dividing large numbers (or searching for a calculator battery). Meanwhile, the subject has wilted, the wind has come up, the bulb has blown, the light has changed...