Hei I have just done one of these here in England, the main prob I can see you having is that to do this properly you need the horse and rider there so you can keepadjusting the flocking to get the fit right, If your just adding flocking to it and really with an old stubben there should be no need to take all the old flocking out unless something drastic has happend to it, you can add more flocking and adjust it without taking the saddle apart at all, if you lift up the flaps you should see two slits in the top of the panel where you can add new flocking. As a general rule of thumb you should be able to get three fingers between the horses withers and the the saddle at the pommel with the rider sitting in place and if you stand behind the horse and look through the centre gullet of the saddle you should be able to see daylight through, then you know that if nothing else at least the riders weight is not resting on the horses spine, hope this helps a bit
best regards
Tony