What is the best amount of DEX to have?

The old rule of thumb was DEX = 2 X LEVEL. But due to all of the DEX and accuracy equips now, you'll have too much DEX. I recommend you get just enough DEX to hit the monsters you're training on and getting as many DEX equips you can. This'll take a lot of planning to do right, though, but I don't think you'd want to have lots more or not enough DEX.