Originally Posted by fatall
I found this on the brooks web site:
Tensioning a BROOKS saddle
There is no exact science behind tensioning a Brooks saddle as everyone’s shape, riding position and weight are different. The last thing you want however is a hammock. When you feel that the leather is bowing, it’s time to tension but only by 90° at a time. Never allow the leather to have bowed so much that it is touching the frame, as this will cause stress to the frame and may cause it to break.
Full instructions on how to maintain a Brooks saddle is available as a pdf download on the section Leather and Maintenance, then Caring for your Brooks.
Straight from the horse's mouth.