That is incorrect. Dex is reliant on your value vs your opponents value. There is no set amount of evade/accuracy for each point of Dex like there is with Luc. Otherwise, Dex would be unnecessary in 1-15. Besides, 20% - 10% = 10%, not 50%.
Opponents with Dex that are the same should experience between 80-85% accuracy and 15-20% evasion.
EDIT: I haven't worked on it in a while (actually PC just reminded me of it

), but
HERE are my calculations with Dex so far.