What is the best amount of DEX to have?

And that's accepting your assumption of 10 hours per level, which IME is not generous at all.

1-30 is <15 hours of playing.
30-40 is <2 hours per level.
40-50 is <4 hours per level.
50-60 is <6 hours per level.
60-70 is <8 hours per level.
70-80 is <10 hours per level.
80-90 is <5 hours per level.
90-100 is <8 hours per level.
At that point if you spend the other 800 hours hitting 120, you'd be taking 40 hours per level which is very generous:f6: Even WKs can do better than that. In all likelihood, it's only around 600-800 hours depending on class. And obviously you don't need accuracy pills the entire time, esp. at 3x-6x you gain a lot of acc all at once and the monster requirements don't change much.