Rear springs

I have a pro extreme blower car that we have redeuce the weight from 2515lbs on the startingline to 2238lbs on the staring line. The wheigt procent is 51% in the front and i used 150lbs rear springs before and it works great, it is shantuff shocks.Now it seems to stiff so the question is what rate should i use for rate on the rear springs when the car is lighter?