I would just start at about 6.5 inches and work your way up.
Its kinda like taking vital signs.When the bow shoots quieter,with less handshock and feels sweet shooting your in the ballpark.
I dont think anyone can tell you exactly where that sweet spot is.Thier is often a recommended range though.Say from 6.75 to 7.5 for example.You have to experiment with the bow by twisting the string up and down for a few days and where ever it feels best at for your draw length,your release and the arrows you are shooting.
I sometimes go up and down a few times before I settle on where I like it how it feels and where the arrows seem to fly best at.
I would bet somewhere between 7 and 7.5 inches on a 58 inch TD recurve with rear/back mounted limbs
I never had a Savage river TD.Thier nice bows though