18mm is right unless you are doing macrophoto with your LF(!!). BUT, that is total range of allowable error. Margin of error either side of the "perfect" position is half that.
Plus, if you plan on a fixed-focus camera, you need as much margin as you can get for depth of field. If you start eating into that margin with errors of positioning the lens, so much lost in quality or flexibility...
Finally, I find it strange to scale the acceptable CoC proportional to the size of the negative. It means you have no better expectations for sharpness (relative to image size) in LF compared to miniature format.
and that is true because you enlarge it less.35mm is most critical due to the relatively high magnification