+1 for Mark's method.

Or skip to 4:40 to hear the math explained and reference to the disc


And there are several smart phone apps that will do this for you. Reciprocity Timer is one such app.

Or explained another way:
Convert the focal length of the lens you’re using to inches. For instance, a
210mm lens is 8”.
8” (210mm) lens, focused at infinity. Bellows extension 8”
· Think of inches in terms of f‑stops: f8.
· Measure the distance from the film plane to the lens board.
· Let’s say the distance is 11”. Think of f‑stops: f11.
Bellows extension 11”, measured from middle of shutter to ground glass.
· The difference between the focal length, 8”, and the focusing distance, 11”,
can be thought of as one stop difference. F8 to f11.

Excerpted from Finely Focused