You measure the "lead" of a chain by the amount of work it proves, not by the number of blocks or speed of blocks. If you based the lead solely on block number

You measure the "lead" of a chain by the amount of work it proves, not by the number of blocks or speed of blocks. If you based the lead solely on block number then an attacker's chain could manipulate the difficulty adjustment to make the attack easier. > Can't I approximate that with 1) the lead that my current chain has over the next competing chain An attacker could keep their chain secret until the attack is sprung. You cannot know how far ahead or behind you are relative to a secret chain. What you know is the amount of work an attacker would have to do, by summing the work proved by all the blocks above and including yours. Once that value gets high enough, you can be pretty confident that nobody is going to rewrite your transaction. The probability that you would have to calculate is the probability that an attacker who has that much compute exists.