Thanks to people in the probability forum who are actually capable of understanding what is being asked, and providing answers...
According to a formula generated in this thread:
http://forumserver.twoplustwo.com/sh...age=0&vc=1
If I managed to get the formula typed in right into excel (I'm fairly confident, since my output matched his numbers for the few examples he did run, but anything's possible, and I've screwed up far simpler things than transitioning a formula into excel)...
With a winrate of 8PTBB/100, and a std dev of 50PTBB/100, the chance of having a 20 buyin downswing in 50k hands is about 4.2%. Over 100k hands, it's 8.25%. Which (assuming the math is even close) basically means that one in twelve people will have a downswing that big in their first 100K hands. So yeah, that seems fairly common-ish. Of course, 100k hands is a *lot* of hands, so this probably shouldn't be a monthly occurence, but if you play enough, you could very easily see a swing that big multiple times.
And btw, this does depend HEAVILY on your actual variance. If the std. dev is increased to 75PTBB/100, the odds shoot through the roof, with an 57.3% chance of having a 20BI downswing over just 50k hands, and a 15.6% chance over just 10k hands. 75 does seem pretty high though, just based on the very few real std. devs I have heard.
Oh yeah, and just because I'm a bitter bastard, I'll say that I'm not at all surprised that these numbers are higher than the risk of ruin with 20 buyins. Which should leave jay_shark thinking something along the lines of...
"What? How is that possible? It can't be higher than the risk of ruin. I "proved" it. I am a probability god, and cannot possibly be wrong? Mortal fool, you dare try to give me lessons in probability?"