How to become a strong Dagger Thief?

When you start, get 4 INT, don't worry about the rest of the stats. Keep doing this or your stats will suffer.

LV10: 10 STR, 25 DEX, 31 LUK
LV15: 10 STR, 30 DEX, 51 LUK
LV20: 10 STR, 40 DEX, 66 LUK
LV25: 15 STR, 50 DEX, 76 LUK
LV30: 20 STR, 60 DEX, 86 LUK

You are the only type of thief who requires STR because your dagger has STR requirements, STR does not affect your damage. DEX should be twice your level, check how many STR the next dagger needs and increase that accordingly, put the rest of your stats into LUK and the amount of DEX needed for the next weapon/armour upgrade.

After Level 40, your DEX requirement decreases. You will need 10 less than usual, so just put DEX in at level 49, 50, 59, 60, etc. Put up however much STR you need for your next dagger. You only need 90 DEX at level 50, 100 DEX at level 60, etc.