Sure there is, but if you need it at 99.0%, you'll have to know exactly what it is right now to dilute it by the correct amount. Your end state is only as precise as your knowledge about it's current state.
And while your end metal might be 99.95% pure, that does not mean your solution is at a 99.95% concentration of Rh2(SO4)3. That would mean that 99.95% of the weight is rhodium sulfate, and only 0.05% is free sulfuric acid. I doubt that very much. The old bottles of plating solution were ~1 ozt/qt, so the buyer knew how much PM he was getting. Another example: when I have 300ml of a yellow gold chloride solution, and I've done everything right, I'll get some very pure gold powder out of it (hopefully .995 or better), but that has nothing to do with the amount of gold in the solution. I can't tell density from color very well, so there could have been 10g of gold in there, there could have been half a gram. I'll know when I precipitate, wash, dry, and weigh it. Until then, I can't tell you what % gold was in the solution. Density would probably be a more practical measure at that point anyway (g/ml3).
