Assuming you're building something a little off the beaten-track, then what facts do you need to establish about the vehicle before you can work out what damper / spring rates you need?

Cheers.

Dry weight, wet weight, laden weight, Purpose & Use, expected terrain.

Thats where i'd start.

unpsrung weight on each corner

Ahbut only 3 corners here I think.

Well all the roll resistance is at the front, just like a monoshock setup. If you are running coilovers either side you can either take the roll out with springs, or bars.
Or both.
Assuming you have a downward facing swinging arm ar the rear to act as anti squat you want to mahe thr rear ride frequency as low as poss say 2hz. Front around 10 to20% higher. Then keeep on adding antiroll bar untill it goes straight on at corners then back off as much as you feel comfortable with.

There is a simple graphical method if you are interested of measuring susp frequency which only requires corner weight scales, a ruler and a preprinted graph

Just swap springs about till the freq is right. On the other hand you can calc it but it requires accurate measurements or the result is poo
Cheers!

You need to know the weights on the front and rear, and then measure the weight of the unsprung weight. Jack the car up and then rest the wheel on a set of scales. You can then have a good idea about the sprung weight.

Now you need to know the damper angle when fitted, and then the leverage imposed on the damper.

From here, you can work out the frequency a spring rate will give, forming a good starting point for actual testing.

HTH

Some discussion here

Cheers

Fred W B