STR + [STR/10]^2 + [DEX/5] + [LUK/5] + WeaponAtk + AtkBonusCards where [] indicates you round the value inside down before continuing and ^2 indicates squaring.
say you have 110 Str in total, your melee attack from str alone will be 110 + [110/10]^2 = 110 + 121 = 231 atk
i.e. you get 121 bonus atk from having 110 Str
EDIT: to clarify that it's a melee atk formula. ranged is calculated by a different one which is DEX + [DEX/10]^2 + [STR/5] + [LUK/5] + WeaponAtk + AtkBonusCards