I think the rake of the chassis will change whether it is loaded or not. Also the weight of the body tub , fuel tank/fuel etc will make a difference.
The factory spec used to be 4 Degrees , but the set up was pretty crude (very little evidence of a jig in the early days) and most seemed to come out around 2 +/-
At my last rebuild (some years ago now!) we set the castor at 6 Deg with the chassis level on axle stands. Quite tricky to measure precisely once the suspension is all built up and the car on its wheels , but I seem to remember the result being about 5+/-. The car tracked well and I was pleased with the result.
