Hi man,
I have thought and searched a lot on this issue, unfortunately it is not matter of calculations and math, it is really a phenomen.
to answer your question, you should first state how much without any supp etc. you produce, none? then what is your treshhold for precumming? some people here said that they did not have any one drop of it and they started precum after 100mg pygeum and some like me with no precum, even after 1000 or 2500mg of pygeum did not secrete it. and those who start after 100mg also said that the amount did not increase after increasing the dose of pygeum, it is somehow the game of 0 or 1, there is a switch, you make it on and then it does not have a dimmer to play with the amount.
I will watch this thread with a big interest and hope that there will be guys who can give the exact formula to do it but I expressed my own experiments and opinions.