Hi Malc,
Off the top of my head I think 5 millimag precision on a mag 15 star might be quite tough for an amateur. A starting point could be the signal/noise calculator here
http://stupendous.rit.edu/richmond/signal.shtml
But there are several other limiting factors (and tricks of the trade) to this sort of transit detection work. I would suggest contacting Bruce Gary who is a (the?) top amateur expert on transit detection.
http://brucegary.net/
Cheers
Robin