LOL nit my ass 24/20 and my stack is in the middle up against quads on a regular basis! No but seriously does it assume that you play like an uber lag style and pay a tonne of rake before you get some decent points like that calculator suggests? I mean I was very happy when it said I'd be making supernova sometime next year or even this year if I put in a lot of hours. Turns out I don't even get enough points to make it to platinum...
The calculator may be slightly biased to give you the impression you can make more than what you could in reality but I just assume that you're paying less rake than average (not sure what stakes you're playing but if it's micros it's almost certainly the case even playing 24/20).
edit: just use the VPP or VPP/hand function on HEM/PT to get an estimate of your VPP rate.
If your going by hours & not hands played, I think the Pokerstars calculator works on the assumption that your playing your max # of tables from the moment you sit down to the moment you get up, which obviously isn't the case, especially if you table select.
Also, make sure your hem is up to date if your using that to determine your VPP/hand. Mine's about 15% less than what hem indicates I make under the dealt rake method.