With a partition coefficient how do you decide which solute concentration is the numerator? Kpc = [X(in solvent A)]/[X(in solvent B)]

I realise that Kpc is actuallly a ratio but when you need the numerical value it has to be treated as a fraction.

When it has to be treated as a fraction how do you decide which solvent is A and which is B?
The proper way to express the partition coefficient of compound X distributed between an aqueous and organic phase, for example, is:    

$$\mathrm{(K_P)_X = \frac{[X](org)}{[X](aq)}}$$ 

This unambiguously expresses the mathematical fraction to be used in calculating the partition constant for a solute, X between two solvents, aq and org.