There's nothing wrong with your calculations - it's easy to get forward of C of G in a PA28; in my old Cherokee 140, a 14 stone pilot and an A/C filled to the brim with fuel put the aircraft on the ragged edge and made it quite unpleasant to fly. Two large people in the front seats plus full fuel was quite a long way forward of C of G.
Either a large sandbag or a 10 gallon plastic container full of water, strapped down in the baggage compartment will transform matters.
Just make sure you strap it down properly to the attachment points as, in the event of an accident, a sandbag or 10 gallon container of water can make quite a missile.
I found 4 cases of wine bought at a french supermarket restored the c of g nicely.
Last edited by wsmempson; 29th October 2011 at 15:03.
Reason: illiteracy