Another reason, of course, why this is not a strictly mathematical issue, is because the precise derived numbers are not, in themselves, critical. I.e. the actual figure, whether it is 0.38% or 0.32% or 0.45%, is not the entire story. It is only an indication of the relative potential for further deviation between the currencies concerned.
A high(ish) number would indicate good potential for more deviation whilst a small(ish) difference suggests the currencies are moving relatively in parallel strength. It is not an exact science what these number are saying. And, of course, one should consider these numbers in the light of the other factors that apply to one’s trading method.
As has been said many times, this is not a “turnkey” trading system, it is a method to determine which currency pairs one might best apply one’s own trading method to.