Given a 3- or 4-frame film strip, with irregular paddings a b and c (optional), and assuming a fixed frame width w, this calculates a value that can be used for VueScan's frame spacing setting to ensure that all frames are captured in their entirety.
For some inputs, no possible valid answer exists. For a 3-frame strip, the following general rules apply: If A >= B, a valid value always exists. Otherwise, B must not be more than twice A.