You need some kind of VTF gauge, just to be sure you're within the recommended range for the cartridge. The $20 Shure balance is fine. After that, VTF must be set by ear. No one can tell you that your PDQ X1000 cartridge must be played at precisely 3.14159 grams. Each rig and each individual cartridge is different. A superaccurate gauge won't help you find your particular sweet spot.
It will help you reproduce it however, that's the only reason I bought a fancy scale. If I need to change my setup for some reason, I can quickly and accurately reproduce a VTF that I already know is good. My cartridge happens to be very sensitive to VTF, + or -.05g is audible. Other cartridges may be different so YMMV.
You can get a better scale than the audiophile ones for a lot less money. The Clearaudio is accurate to .1g and costs $400. I use a MyWeigh MX-50, which is accurate to .01g and cost me $95. Ten times the accuracy, one quarter the price. There's a catch or two however.
The MX-50 and other small jeweller's scales are highly magnetic. You can't plunk your stylus down on one and get an accurate reading, and you might even damage the cartridge. They also aren't designed to weigh at the plane of a record, which is necessary for accuracy.
On my TT solving these problems was simple and cheap (free actually). I place the MX-50 on the table next to the TT and stand a homemade, flat-topped tower on it. The top of the tower is the same height a record would be and that's where I drop the stylus. I made the tower out of a sheet of paper and 2" of scotch tape. Since my TT is nearly 8" high, this keeps the cartridge well away from the magnetic attraction of the scale. I don't know if you could rig up something similar for a P3 or not.
