While the traditional "Three Isthmus Border" would be the Finnish maximum demand, I personally think that we should not assume that Finland gets it by default. One major issue is Murmansk and the Murmansk railway - if Germany decides to go to the effort of depriving Russia of those major assets, then we might see that maximum Finland realized. But if Russia has at least some say in the issue, and Germany for some reason does not want to waste "diplomatic poker chips" pushing Russia in this area but rather uses that leverage further south, Russia might keep the Murmansk and railway combo and in that case realistic maximum Finnish borders would stop just short of the railway line.
In any case we need to remember that for Germany Finland and Karelia are more peripheral areas than Ukraine and the Baltics, and thus, if it is necessary to choose, Germany would sacrifice some of its (and Finland's) goals up north if that means getting better rewards down south. This area is strategically much more important for Russia than for Germany, especially when the Russians are losing most of their Baltic ports, so they would try to hold on to Murmansk and the railway if at all possible. In most cases, I believe that Russia would not be in a position where Germany can dictate all the terms of the peace, but even if Russia is in dire straits it would have some chances for bargaining at least. Perhaps in some peace deal, just the ability of potentially quickly cutting the railway and rendering Murmansk useless as a port, by having the Finnish border right next to the railway line, might be enough for the Germans in strategic terms.