Chance to Hit Formula:
200% * alvl / (alvl+dlvl) * AR / (AR+DR)
Basically, there's just 2 comparisons. First compare your level to the monster level, if they are the same, then ignore them. Now compare your ARating to the monster's Defence, if they are the same, then you have a 1/2 (50%) chance to hit. If your ARating is double the defence, then you have a 2/3 chance to hit. If you're tripple their defence, then it's 3/4, at quadruple it's 4/5.
If you half the enemy defence, then you essentially double the top and bottom of the fraction, but then reduce the bottom by 1. So 1/2 becomes 2/3, 2/3 becomes 4/5, 3/4 becomes 6/7, and 4/5 becomes 8/9. In percents, this is 50% to 66%, 66% to 80%, 75% to 86%, and 80% to 89%.
No matter what, you'll miss at least 1 in 20 swings and at most 19 in 20 swings. As a quick but mildly innacurate shortcut, the -50% def of Eth rune will cut your chances of missing in half. If you would normally miss 1 in 8 swings, you'll now miss about 1 in 16 swings. The innacuracy appears mostly in the lower numbers, but if you're missing 1 in 2 swings... you need help