Re: Using DEX instead of STR for precision attacks.
Is the feat he wants called Weapon Finesse?
If it is, I think you should buy STR equal to the value of the character's DEX with Inherent and 0 END Advantage as this STR should not be subject to adjustment powers nor should use any extra effort to use over his normal STR, Linked(proportional to DEX), Use normal STR score for STR Min, END use (-1/4), Only for Hand-to-Hand Damage(-1/2), & Only Unarmed or One-handed weapons(-1/2).
The idea behind it is that it acts as a STR replacement for your appropriate weapons(Weapon Finesse can only be used with a light weapon[unarmed is considered light], rapier, whip, or spiked chain made for a creature of your size category), can only be used with damaging your opponent and still requires the character's base STR Min to wield the weapon. Drains on DEX will still affect how much of this power's STR is used because of how linked works and it still respects the way Strength is used in the Hero System.