The math is fine, but it's still actually wrong. You're assuming that any offense in the NFL, against any defense, in any situation, will be successful at the same rate. That's stupid.
Cleveland Browns, down 14 to the Chicago Bears, better kick the XPs.
I very clearly stated that it assumes no other variables are involved. As for your example, I think quite the opposite
may be the right decision. Assuming Cleveland is about average in converting, they have about a 40% chance of winning (assuming they score twice). If they take the PATs and go to OT, do you think they have a 40% chance of beating the Bears? If not, go for two. If Cleveland is well below average but has a kicker that can regularly boot a 50+ yard FG (for OT), then take the PATs. Now, that's just one variable. Home vs. away is probably a consideration as well.
And, Marc finally admitted that it is an emotional decision, not one based on statistics. No one remembers when a coach goes for two and wins. No one forgets Tom Osborne and Nebraska in the Orange Bowl 20+ years later.