Ask Flex-A-Form to do the rate factoring. If you want stock-rate, ask Flex-A-Form to make the springs accordingly. I would not guess at what the new rate should be

Remember that the stock leafs are too stiff for a smooth ride with only the driver ... they are designed to carry a full load (about 400 kg of passengers and luggage)

Lowering blocks wont help. At 3" lowered the axle bump stops are already hitting the body