I still think the question is how much of Ukraine will Putin settle for to leave the rest of Ukraine alone. How much for a win and how much for a loss?
Figure out the range of a tactical nuke missle on a mobile platform add 150 miles. Then find the line. That’s minimum and remember ICBM’s aren’t in the mix. That’s why they call them Inter-Continental.
Ukraine likely splits with Russia at the river and the western Ukraine as the buffer state that is Putin’s (reasonable) aim as he has identified the past decade or more.