Ah, I see, given these estimates, I can understand why this does not look like a Pascal's wager to you. It seems to me that the odds look so grave to you because you gloss over several steps during this potential escalation. For example, "Russians intervening militarily" could be anything from posturing to weapon shipments to a surgical strike to a Czechoslovakia-style tank-roll or Afghanistan invasion. My guess that the odds of the latter is below 5%, as it has not happened since (Chechnya was the closest case and that was a Russian territory actively hostile to Moscow). Similarly for every other case. Note that the scenario you describe (Soviets invading and losing) happened in a much more tense atmosphere in Afghanistan, and there isn't even a hint of the Soviet leaders at the time seriously considering nuclear escalation. So, the historical reference classes do not seem to bear out your estimates.
It seems to me that the odds look so grave to you because you gloss over several steps during this potential escalation.
Have a look at this: http://www.zerohedge.com/news/2014-02-28/ukraine-acting-president-says-russia-starts-aggression-against-country-russian-plane Original source: http://www.pravda.com.ua/rus/news/2014/02/28/7016674/
Posturing or not, if this info checks out then by existing treaties USA and UK are obliged to help Ukraine against Russia. You see, Ukraine gave up nukes in exchange for safety guarantees from Russia, UK and USA.
http://en....
A Dan Carlin Podcast about how the United States is foolishly antagonizing the Russians over Ukraine. Carlin makes an analogy as to how the United States would feel if Russia helped overthrow the government of Mexico to install an anti-American government under conditions that might result in a Mexican civil war. Because of the Russian nuclear arsenal, even a tiny chance of a war between the United States and Russia has a huge negative expected value.