Today I would give myself a 1 in 20 chance of being wrong. [...] I would probably misunderstand them at least 1 time in 20.

So far so good.

If I convert that into "There is a 5% probability that the God of the Bible exists and will send me to hell"

Don't. That's a mistake. There's a 5% chance (say) that you've seriously misunderstood some of the things that Christian theologians think and why they think them. That's not the same thing -- it's not anything like the same thing -- as a 5% chance that they are right about Christianity and you're going to burn in hell for getting it wrong. It's perfectly possible that both they and you are wrong, and unless your reasons for being an atheist are much worse than I expect that's the most likely state of affairs conditional on your being wrong.

I bet there are a lot more than 20 groups of people with mutually inconsistent beliefs, whose beliefs you have at least a 5% chance of having seriously misunderstood or otherwise failed to refute perfectly.