That’s a great question. My guess is that it’s fairly consistent if you consume enough grapefruit juice. If you consume just barely enough grapefruit juice to increase the bioavailability then the bioavailability could vary quite a bit because maybe you drank x amount of grapefruit juice one week, then next week you drink again x amount, but maybe the juice you drank the second week is from a different batch of grapefruits, or is of different age or source. There are many variables that can influence how much of the compounds that improve bioavailability are in x amount of juice. Therefore, if you want more consistent effects it’s wise to drink a bit more than the minimum you would need.
This is the case because there is a threshold effect whereby x amount of grapefruit juice of a particular batch will lead to almost maximal increase in bioavailability. Consuming more than that will not increase the bioavailability significantly more. Lets say that for one batch of grapefruit juice the amount that leads to maximal inhibition for you is 200 ml. It would be good idea to consume 300 ml each time, instead. That would make sure you’re likely to still get close to full inhibition even if the juice you buy next time contains 50% less of the compounds that improve bioavailability.
One more important thing. Regarding the predictability. If you drink a high dose of grapefruit juice every time you take rapamycin the amount you absorb might actually be less variable and more easily comparable to that of other people. One of the main reason people get different levels of rapamycin after ingestion is that they have different amounts of the CYP enzymes in their digestive tract. But two people drinking a lot of grapefruit juice will both have much lower levels and therefore will likely have more similar bioavailability. So I think drinking grapefruit juice with rapamycin will tend to reduce the variability between individuals in absorption.