I tried it using 4 scales and found myself getting frustrated for a while. I kept getting different readings. I was told to disconnect the shocks and let the chassis rest on the suspension. I tried that and felt a bit more comfortable. I think the scales really narrow down the balancing. With the scales the slightest differance in battery weight, battery bars, or even where you route your wires changes the scale reading drasticly.
Then I tried the hudy balancers like the ones shown by Marcos and my car was balanced. If the hudy tools works for everyone else then that is fine.
When I changed over to Brushless my car was way off balance and to this day I am still trying to use the Hudy Balancers only.
I have since sold my scales here on RCTech. They work but it takes longer to get it done.
Do not adjust by changing the preload because the car would still be out of balance. That would help for tweak only. To balance try moving electronics from one side to the other or adding weight.
Just keep in mind that the scales must be on a perfectly flat surface and that the chassis is not tweaked before you start. Otherwise your time spent on balancing was to no avail. Hope this helps.