So in this kind of situation it might be benefical to make the upcoming pocket smaller so that units have no room to retreat (or rout inside the upcoming pocket)?
If your ports on Lake Ladoga is operational, your units will rout out of the Leningrad area and land somewhere east of Lake Ladoga if there are no town or cities within the pocket to rout to.
I would imagine they would take some real casualties from withdraing that distance, but I have never tried this.
Just remember that if you stack your own units three high in every hex of the pocket, you have also denied yourself the opportunity to replace weak units on the line. Also, stacking three high means that a savvy German player would make it a point to take or destroy Osinovets, and then your units suddently have nowhere to rout and will start surrendering instead.
If I were you, I would consider evacuating what units you can over Lake Ladoga while holding on to Osinovets as long as possible (what level fortification does it have and what do the Germans have across the Neva?). Of course this also depends on the turn number. If you are at turn 16+ in a bitter end scenarion, I would consider holding on to Leningrad with everything available instead.
We need only to kick in the door, and the whole rotten structure will come crashing down.
Adolf Hitler, on the eve of Barbarossa.
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