I agree. Terminology is key.
I know potency to be the strength/power of a substance because effect is subjective not only the person but also the conditions of that person (fatigue, nutrition etc).
So when a strain in measured at 22% lets say... how that effects person A on one day is not the same as another day and that is dependant on how much rest or food the person has had. Weather conditions also play a role as that affects a person body chemistry. Now whether the difference is large enough to notice is another matter. I know friday after a long week of work a couple pulls on my Pax will crush me. Whereas on a Sunday afternoon i have to suck back half the bowl for the same effect. The potency of the resin has not hanged. I have... and therefore the effect has. Or at least the perception of the effect has.
You're spot on about individual (and cross-individual) variability and the multitude of factors that can contribute to a drug response. I've always been fascinated by the idiosyncratic paradoxical response of excitement that can be seen with sedative herbs like Valerian and Kava. As you point out, timing of a dose can also be crucial. Personally, I can't wakeup in the morning without large quantities of coffee...yet if I drink it in the late afternoon, it puts me right to sleep (adenosine levels/binding?). Then there's the classic biphasic dose-dependent response, where you give a small dose of a substance to produce a primary effect and a large dose to produce the exact opposite effect (bell-shaped dose-response curve see in toxicology).
Although the euphoria and other common psychoactive effects of cannabis are mostly subjective (excluding use of MRIs, etc.), when it comes to potency, there are plenty of ways outcome can be objectively measured. As a general example, we could measure the 'potency' of two antihypertensive drugs, by comparing the effect of an equal dose of the two drugs on lowering blood pressure. Likewise, many outcomes could be measured with cannabis (e.g. migraine/epilepsy frequency, sleep latency period, inflammation blood markers). Basically, if you can measure it and get a baseline reading, some measure of potency can likely be gathered.
Anyway, thanks for the conversation. Much appreciated.