So it basicly was his best case scenario and if France would intervene, they would probably get less.
I don't think Nappy was very concerned about Austrian acquisitions form Prussia. His obsession was with Italy, and in particular with ensuring that Franz Josef didn't wriggle out of his promise to surrender Venetia.
I can imagine him mobilising on the Rhine, as a warning to Austria that if she tried to keep Venice, she wouldn't be allowed to acquire Prussia's Rhenish provinces as well. He might also take the opportunity to pinch Luxemburg or even conceivably parts of Belgium, but that is far less likely.
Iirc my AJP Taylor, he had indicated that he would accept any Austrian gains which "did not upset the European balance of power". In practical terms, this probably meant Austria could acquire German territories equivalent to her Italian losses (incl the prospective loss of Venetia), so that Austria herself would get Silesia in lieu of Lombardy and Venice, while the deposed Grand Dukes of Tuscany and Modena got new states, probably on the Rhine, of similar size to their former ones. The Habsburgs, IOW, would have total territories similar to those of 1858, but further north. What was left over would get shared between Bavaria, Wurttemburg and Hanover, while Saxony would probably regain some or all of what she lost to Prussia in 1814.