Estimating rear spring weight.

[donraye]

Hi all, I bought a ride height adjustable coil-over conversion kit for my car and it come with rear springs of 150 lb./in. weight. I am about to put the kit in and am wondering if that spring weight is about right for the car or if I need to go lighter(maybe 125 or even 100lbs). The car is a Street Beast Cobra with a stock Mustang 8.8" 4 link and I am guessing the total car weight to be about 2,500+ lbs with gas. With me and the wife would be anonther 345 or so. Do you count the gear and passengers when you figure this or not? Do you simply take half of the total wet weight of the car to figure the rear weight or is that all wrong? Can anyone give me an idea of what spring weight would be a good place to start or how to estimate this? The car will likely not be used for competition driving, just street use. I would hate to put these 150 lb. springs in and then have to change them out right after. I am not the worlds best mechanic to say the least and it will probably be challenge enough for me just to do this once, let alone a second time right away. I have no way to actually weigh the car at the rear so what is the way to make a good guess? Any help would be appreciated!
Thanks, Don