I would assume that like North Korea nukes are only useful if not used. Once those weapons are used (if they are in a state to be used) there is nothing keeping Russia viable as an independent state. It would be the starting gun for a larger conflict as no nation with any ability will let even the smallest nukes to be set off without a response. Those weapons are doing their job only if not fired and might be the only "red line" this war truly has.

Under 0.5 nukes used: 1/1000000000 Over 0.5 nukes used: 1000000000/1

No-one, not even Russia, is going to cross that line. This is a regional conflict with casualties in the tens of thousands. WW3 was a global conflict with casualties in the tens of millions.