1803
In the story "the only man in Prussia" the Kingdom fortuitously gains the lands Russia acquired after the second partition of Poland. Mostly due to Russian mismanagement and squabbling, the Russians are defeated in detail, ending with the Czars capture. Prussia dominates the river lines of eastern Lithuania and awaits the Russian responses.
A war with France ensues that leads to the Prussians (North Germans) stepping i after an Austerlitz and defeating a worn out French - then March on Vienna. By now it's 1805 and Prussia fairly dominates North Germany and Lithuania- nappy is killed early in the story and Morau is leader of France. Great Britain is free to respond to situations in the Americas, and Russia is on its knees not only because of Prussia but also the wars against the ottomans and Persia.
I have to admit that it's a bit of a Prussia/British wank, but hopefully told with humour and well researched facts. What I need is not a military rebuttal; but a political reasoning as to how far Prussia can push East and South without it becoming untenable in politics/diplomacy. Feel free to rubbish the ideas. However, the campaigns have been tested in models over the years. A lot of work and time has gone in to working out the possibilities of each of the situations. It's the diplomacy I'm short on. ( especially the American situation at this time)