I cannot say what the "correct" amount of tension is because it is specific to the weather on any given day and I am still learning this myself. However, it is normal for your mast to bend when significant downhaul is applied.
The best explanation of this I have seen is in one of Rick White`s videos. Essentially, the luff of a sail is cut as a curve. It is not a straight cut as seen when the sail is laid flat on the floor (without battens). When the sail is then put onto a mast - which of course is straight - this causes the sail to assume a full or airfoil shape (in cross-section).
Applying your downhaul IS supposed to bend the mast. This allows the sail to assume a flatter shape because the "bent mast" better conforms to the curved cut of the sail`s luff.
Flatter sails are generally desired in heavier winds because a flatter sail has less power that a fuller sail.
Hope this helps,