Hi _Archie_ !
Welcome to C&V...

I've never felt the need to retension one of my saddles - but from what I understand, when the saddle has many miles on it it may need to be tensioned in order to remove the sag.

So though I have no experience with building saddles or even in tensioning them, I would think that this same logic would apply to a new (or re-built) saddle: ie: It needs to be tensioned enough to prevent sag.

- Sorry I could not provide a specific number - but consider too that leather is a natural product with a great deal of variation from one piece to the next, so it is entirely plausible that there might BE no single spec. - just the experience and the judgment of the craftsman who assembles it.

This website provides some additional information on Brooks saddles that I thought was interesting.