My method is to measure the bellows extension, from the ground glass to the lens board, which, believe me is close enough, then divide that number by the focal length of the lens, square that number, which yields your Bellows Extension Factor, BEF, then divide your ISO by your BEF and set that into the meter and take your reading. Being super precise is not usually necessary, although when shooting chromes it can't hurt.

Example: 11 inch bellows extension, 8 inch lens, = 1.375 X 1.375 = 1.89. ISO 100 divided by 1.89 = 52.9, for a working exposure index (EI) of 50, which in this case will yield a one stop correction. All of this will keep you in a 1/3 f-stop tolerance, an industry standard.