I think i have made a rather big mistake with my rogue. Hes lvl 20 and I thought that getting the feat Weapon Finesse would mean that the damage i do would be based of the amount of dex i have (39) however the amount of damage i do is about 20 per hit when facing an enemy. Have i got the description wrong?

You can use your Dexterity bonus instead of Strength bonus to determine bonus to attack with certain weapons if it is higher.

Any advice would be great thanx