The formula you worked out is wrong the formulas are as below and don't work when rewritten as you tried:
- Monster’s chance to hit: (Monster’s Attack Bonus + 10.5) / (Target’s Armor Class * 2)
- Player’s chance to hit: (Player’s Attack Bonus + 10.5) / (Target’s Armor Class * 2) + 25%, rounded to nearest 5%
All I didn't say was of course you always miss on a 1, sorry tired I shouldn't presume people have that as a given.