As Mince says, there is a lot of info on the forum on springs but in my opinion progressive springs dont really deliver what is expect, they have too little spring rate where you want it reasonably firm in braking on smooth surfaces and the like
if you are really good you can use the dive to your advantage but for mere mortals a linear spring will give you linear performance of the spring which is better in my opinion
you can also use oil weights and air gaps to tune suspension with linear springs but this is much more difficult when you don't have a constant spring rate
I would suggest getting linear springs to suit your weight, get the sag right and then adjust the oil weight for the damping you want
Racetech have a calculator
http://racetech.com/