So I have tested setting it up again today on 4 scales, and found some interesting results.
I found out that it doesnt read consistent because of excessive play and slop on suspension arms, hubs, pivot balls, axles, etc... Well, its a 2 year old car with lots of hard racing.
When I roll the car, everything changes. I hope that with a new and well maintained car, this shouldnt be as much as it is now.
Second thing that I found out that it reads the fr+rr or fl+rl same ALL THE TIME. So you're right its a better if not the best way to balance the weight of the car.
Here are some examples: (no different adjustments made, just by different compressing of the suspension)
FL: 373 FR: 327
RL: 411 RR: 424
FL: 372 FR: 330
RL: 412 RR: 420
FL: 364 FR: 334
RL: 420 RR: 416
So I have a question: How can we know that we put the car on scales exactly same compressed all the time? I mean the settling thing of the shocks, its uncontrollable.