You need to deduct the weight of the rear end with tires. (about 300#) So 1500 divided by 2 will be 750# per spring. 5" total preload with 150# spring AT RIDE HEIGHT.
The amount you preload will be based on how much travel you have for your shock. I wouldn't use anything LESS than 7" of travel for street. 3" compressed/4" travel left. You want the tire to be able to travel down into the potholes to reduce the severity. Otherwise, you got a buckboard that will throw you sideways with every tar strip you drive over.