I understand your point, but I think you may be missing the point!
It is my understanding that Exential 'promise' nothing. They have apparently achieved between 7-9% on average per month for the last few years.
Unlike other 'ponzi' schemes, Exential encourage you to withdraw your returns every month as there is no return paid on 'profit' only the contracts you have - this doesn't really fit with the classic ponzi model (such as F1) which encourages you to leave all money in the fund, thus ON PAPER you appear rich!
Clearly then your 'compound' calculation is not relevant in the case of Exential, although it may be true of other 'similar' schemes I'm hearing about!
Incidentally, I'm neither a promotor nor detractor of this scheme, but I do hear that guys HAVE got their returns this month despite a thread gloating to the contrary!
If any of you have HARD factual evidence that Exential is a DEFINATELY a ponzi, please share.