Bolting on a neck dead straight/centre

Hey guys,
Im sure this questions has been asked in the past but after scrolling through I couldnt find a relevent thread so apologies in advance.

I have an IB-5 kit currently being built and im am nearlly ready to screw the neck to the body. The pocket on the body is slightly loose for the neck to make a tight (and more importantly straight) join. The angle of the neck changes a lot when moving it side to side so my question is:
How do I make sure that the neck is centred to the body and straight? With as little margin for error as possible?

Ive done a bit if thinking and i am fairly sure the best way would be to mark the centre line on the neck and the centre line of the body and line both up against a long straight edge.
Once ive done that ill clamp it in place and shave down veneer until it will tighty fill the gaps on both sides of the neck to make a tight fit.

Any feedback or tips would be much appreciated, Thanks! :)

In the 'How to...' section there's Bridge placement on a bass, which covers neck and bridge placement/attachment.

And, for a slight variation, here's how I did it on a recent bass build: Attaching neck and Attaching bridge.

Different kits and hardware, but should give you the general idea.