Given what we've been talking about recently at various threads, such as
this one, we should try μ = 9 or 10. Now everything up to nonary or decimal is trivial, everything up to heptadecimal (and enneadecimal) is the same, but with μ = 9, octodecimal now has {0, 1, 2, 3, 6, 9, c, f, g, h} easy, making a figure of 78.9% (171 - 36 = 135), and with μ = 10, vigesimal has {0, 1, 2, 4, 5, a, f, g, i, j} easy, making a figure of 73.8% (210 - 55 = 155).
Tetravigesimal still has only {0, 1, 3, 4, 6, c, i, k, l, n} being helpful, resulting in a much lower figure of 65.0% (300 - 105 = 195). I am not sure how to measure the length of the line as a factor, though. It seems clear that past 20 this is a serious problem.