I have a Shakespear X-18 Wonderbow 52″ 45@28. I shoot 55-60 spined Douglas Fir shafts w/145 gr tips and the brace height is 7 1/2″, maybe a little more. Quiet (reasonably) and no strange vibrations. It didn’t like anything much over 8 1/4 or 7″. I don’t know much more other than a general brace height for recurves is 7-9 inches. I think I read that in “Shooting the Stickbow”.
I also have a Shakespear Ocala #55#@28 60″. I can shoot the same arrows out of it but the brace height is about 7″ and the bow doesn’t seem smooth. I switched to 70-75 spine and now the brace height is 7 1/2 also. Seems to feel better. Hop this helps.
Craig