OK, so now I understand. Having separated, even though you store both separated oil and aqueous base in the same container, you take advantage of the fact that they occupy two separate layers to decide what fraction of each to add. It can't give you the accuracy of control that skimming the oil off and storing it separately would give, but nonetheless it seems to me to be very probably the same technique that BIRs might use, in just the same way that they use one spoon for (virtually) everything whereas we tend to have our 1/4, 1/2, 1 teaspoon, 1 tablespoon, etc measures -- i.e., they favour simplicity over complexity for obvious reasons.
** Phil.