If you want to handle all locks and traps in the game then 100% is enough. So I believe that with 18 in dexterity that would be dualling at level 7 (I'm assuming thiefs get 20 points per level in IWD... can't remember).
If it bothers you to dual so late you can give some 80% in each. That would be enough to open most chests and disable most traps. And for the rest you can easily use knock spell for locks and avoid or simply save and set off traps.
So I think it all depends on wether you playing HoW. If yes, I would go for 100%. You get plenty of XP, thiefs level up fast and it will save you later a lot of hassle if you try the harder HoW locks and traps. If you play IWD only, consider dualling earlier, otherwise you might spend most of the game without your thief skills anyway.
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ZFR
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