I thought of that, but decided that the OP wasn't taking it into account, and so the error was worth pointing out (which turned out to be correct). On the other hand, I don't see how one can establish a linear scale of ability. IQ measure, for example, is often defined based on calibration in a form "1 in X", and then giving, say, 16 points above/below 100 for each standard deviation to the area in normal distribution weighting 1/X. This also allows to cross-check IQ tests with other g-factor tests, competitions, etc.