Edited

Ok,
as a first experiment the one I have suggested is a good place to start because it will give you an idea of what kind of benefit (if any) grapefruit has over lemon.

However you are correct that ideally one wants a trial with many people (to ensure that there is not something unique about the volunteer). But that is problematic. Where are you going to get all these volunteers from? Are they going to be selected randomly, or are they going to be self-selected? What are you going to get them to do - i.e. what protocol will you follow? How are you going to remunerate them for their time/effort? How are you going to guard against malicious interference? Are they all going to be flushing at the same time, or are you going to let people choose their own times? (you should know that flush success has been reported to correlate with moon cycle). How are you going to get them to objectively measure their results? If it is counting then you should know that this is a wildly inaccurate method. If it is stone mass then how are you going to get people to measure this? (I have only known 1 person who was dedicated enough to do this). What are you setting out to test Grapefruit vs lemon, or other questions too? How will you motivate people that this is a worthwhile experiment?

It seems to me that any experiment involving more than a few people will be horrendously difficult to organize. At the end of it your methodology will be open to attack, which you will have to counter and defend (as any scientist does).

It is a lot easier to attack other peoples ideas, then put your own ideas out there and defend them.

JV.