It comes from the formula

Scale at given latitude = scale at equator x secant of latitude, or more simply scale = equator scale / cos (lat).

Therefore if scale is 1:90 at 45deg, then (1/90) x cos45 = equator scale (=1:127.3 approx) and so use (1/127.3)/cos30 to get the scale at 30deg. (Answer is decimal on calculator, use 1/Answer to get distance).