You can still access an archived but fully function version of this page here: https://web.archive.org/web/20181231045818/https://people.xiph.org/~greg/attack_success.html

You can play around with the numbers yourself to understand things. In the top box enter the proportion of the hash rate that an attacker has. So if they have 10%, enter 0.1. For 35%, enter 0.35.
In the second box, enter the number of confirmations they are attempting to reverse.
Multiply the result by 100 to get the chance of an attack being successful.

So if you enter 0.1 and 6, you get 0.0002. This means that an attacker with 10% of the hashrate would have a 0.02% chance to reverse 6 confirmations.