Band size from a gel, against a marked ladder

Calibrate a gel lane against a marked DNA ladder and read the size of any other band from how far it ran. Migration distance is approximately linear in log(size) over a gel's resolving range, so a least-squares line through the marked rungs interpolates between them — and NOT outside them, which is why a band beyond the marked span comes back flagged extrapolated rather than quietly clamped to the nearest rung. Reports R² and the per-rung residual, because one mis-marked rung (a doublet marked as one band, a mis-clicked centre) shifts every size on the gel and a single R² hides it. Distances may be in pixels, millimetres or anything else, as long as they are all measured the same way from the same reference line.