Place potatoes in large saucepan; add enough water to cover. Add 3/4 teaspoon of the salt. Bring to a boil. Reduce heat to medium-low; cover loosely and boil gently for 15 to 20 minutes or until potatoes break apart easily when pierced with fork. Drain well.
Return potatoes to saucepan; shake saucepan gently over low heat for 1 to 2 minutes to evaporate any excess moisture.
Mash potatoes with potato masher until no lumps remain. Add margarine, pepper and remaining 1/4 teaspoon salt; continue mashing, gradually adding enough milk to make potatoes smooth and creamy.