My intention was to describe a time series which should correlate with XMR USD over some local time window. the time window would have to be short enough so that the approximative use of a ratio spread instead of a subtracted spread did not lead to excessive distortion, and long enough for constant noise not too interfere too much with the correlation. The series would need to be scaled by some constant factor to have any utility for supply/demand price estimation. The use of a ratio spread was motivated by a desire to avoid calibrating a scaling parameter.