[kevincox]

> finality which is 6 blocks

There isn't such thing as "finality" in bitcoin right? The probability of another chain growing longer just dramatically shrinks each extra block mined (as most of the mining power will quickly shift to the longest known chain).

There is also a lot of historical data as the bitcoin blockchain has been running for many years. How many times has a chain with a 2-block lead been surpassed? It seems like the data is out there to create a pretty accurate graph for by-chance reversal for a given number of confirmations. (Intentional reversing is a bit of a different issue but you should be able to estimate the computation costs as well).