Re: Loose neck pocket. Doh!
You need some way to get two straight edges between the the bridge saddles and the nut. (Two old high E strings would do.)
The idea is that the straight edges should run pretty much in parallel with the edges of the fingerboard and a sensible distance in from the edge.
With the sacrificial strings slightly tensioned, move the neck heel from side to side within the pocket until the string edges appear parallel to the fingerboard edges. Mark this position in pencil on both the neck and body.
Slacken and remove the sacrificial strings. Offer the neck into the pocket again. Line up the pencil marks. The gap(s) to either side of the neck heel will show you what size(s) of shim it is going to need.