Y is the scaling factor, x1 is the baseline, n is the scaleup, and a result of 1 is perfect scaling and 0 is no scaling at all, then.
Y == (X - x1) / (x1 * (n - 1))
Example... (x1)
(n)
(X), then scaling = (1.5 - .5) / (.5 * (4 - 1)) = 66%
(X), then scaling = 100%