Even lighter yet.
I would start with 15.
And of course it depends upon your plans for the revolver.
If you are going to shoot it a coupla times and then hang it up in favor of a new revolver, you may want the experience to be a little more emotional. (22 Gr)
I think (and I mean I only think) that your first indication of a heavy load will be marks on the ring on the recoil shield engraved by the back of the cylinder. Upon discharge, the cylinder is slapping against the recoil ring and denting it in the shape of the cylinder. You should be able to shoot for a long time at 15 and not get this effect.
That revolver looks in pretty good shape. You can tell if it has been abused by examining that recoil ring before you shoot it.
My reading of history convinces me that most bad government results from too much government. Thomas Jefferson