Oh, sorry, I just realized you were wondering about how much to apply
I looked up the concentration of diclofenac in Voltaren, and it appears to be either the same, or a little higher (about 10% higher) than Surpass. So I would probably use the same amount as Surpass. I think it should be a 5-inch ribbon per joint?
Obviously, it's better to talk to your vet about it.