Martini,
The spring rates for a car have to be looked at as a system--so the rear rate you want is determined by the rate you are using in the front. If you make the rear too soft in comparison to the front for the amount of weight on it, the car will understeer. If you make them too stiff, it will oversteer.
I believe the rate is proportional to the amount of weight on that end of the car--for example, if the car has 60% of its weight on the rear and 40% on the front, the rear springs should have a rate 1.5x the front. At least that's what the spring and shock manufacturer I was talking to got from his formula.
So the spring rate for your rear will be 275 x weight on rear axle / weight on front axle
Jim Oddie