I'm checking some calculations and on-camera indicators I made a while back and need help with a calculation. Before recommending that I go to the home page or other other sites, please understand that that's what I did to get what I am about to present. I struggle with math and am asking, not for a way to calculate bellows extension compensation factors easily in the field, which I understand, but simply for the missing equation for the Excel sheet I already have, which looks like this (click to enlarge):
I have rechecked my formulas at Ken Lee's valuable site; they are fine as far as I can tell. What I would like, is a formula to fill in the last column, giving, e.g. 1/3, 1/2, 3/4, being the increase in stops that I need for that extension, not a multiplier for the time. I realize that it may include fractions like 2/7; I can deal with that.
For any who really wish to know, I use a little stick-on label on my Tachihara bed, which indicates how many stops of exposure increase I need when the end of the front standard baseboard lines up with a number on the mysteriously designed ruler. (I realized today that my sticker's off; I erred previously in my calculations.) This is a quick technique recommended here and elsewhere, which suits my work well; I only need a few values and can interpolate between them. I have only two lenses, so one sticker goes by the left ruler, the other by the right.
Thank you.
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