I found some good options to replace the rear weak coils that come with my completely dead Nivomats (that will also be replaced with standard shocks)but I am not sure how strong they should be:

In this site:
http://www.stockcarproducts.com/springs4.htm
they sell 3 options:
they are all 5"x13" coils and the 92-960 needs 10" of compressed coils to get a normal look so I need the coils to compress 3 inches when loading the empty car, and this car weighs 2300 pounds in total, I guess the front coils will carry like 70% of this weigh but I am not sure.
I don't know how to weigh each corner of the car, does each tire hold the same amount of pounds (1/4th of the whole car) or the front ones suffer more due to the engine and tranny?

Well, the coils rate options are:
125 lb per inch,
275 lb/in and
300 lb/in

I would like to have a normal ride for 4 persons (wife and 2 sons)and I was thinking of getting the 125 lb/in rate... which will need 3"x125=375 pounds each coil to loose the 3 inches I need...that means the car needs to have 750 pounds of weigh in the rear side which is like the 32% of the entire car.

Can anybody please have any ideas or experience on this topic?
I don't know what's the rate of the existing coils that come with the Nivomats.