IR focusing formula

I have seen a question in the "alternative" forum, about IR focusing shift. It made me think that I have a couple of Kodak IR 4x5 packs in my freezer and intend to use it someday. Does someone have a formula that will help me focus my camera? For example, when I am shooting with a 150mm lens and my bellows are 200 mm long when I am focused for the visible light rays, hoa will I find the correct bellows length for IR ? Must have something to do with the IR wavelength and the lense's diffraction factor, but how do I calculate it ? Don't tell me to use F45 and shoot it like I see it focused.

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