Pascal,
Forgive,the dumb question, but were there shims on the bottom of your axle when you took it apart? If so, how many we're there? It's not a 100 percent accurate, but I don't think a lot of people knew that you can pull a shim in order to increase the pre load, this was Sometimes done to delay a kingpin rebuild. So if you have a stack already there, it might be the correct amount.
There is also a way to check the pre load, once the kingpins are installed and the knuckle rebuilt, before you put axle in you can measure the amount of tension with a large hook scale, it should be about 16 pounds if I remember, it's detailed in the universal repair manual. You adjust it by adding or removing shims. Make sure the steering is disconnected when you measure it.
Todd