I do "complete" or "partial" reflocking of English saddles. I was taught and studied under Master Saddler David May. David has a complete tack repair business in Penrith England. I only use "virgin wool" for saddle reflocking, providing the best comfort and temperature dissipation from the back. The material will cushion the horse's back better than unnatural foam.
If a tree is the proper size for a specific horse the wool flocking can be custom "tweaked" to enhance the comfort of the fit for the horse. This is based on the on-the-horse inspection (of the saddle) and the specific set of templates that provide objective criteria of the horse's confirmation (as explained in saddle-fitting).