This is from Robert's Rules (Bob Ciaffone) on No Limit Poker

Section 14 No Limit and Pot Limit:

3. All raises must be equal to or greater than the size of the previous bet or raise on that betting round, except for an all-in wager. Example: Player A bets 100 and player B raises to 200. Player C wishing to raise must raise at least 100 more, making the total bet at least 300. A player who has already checked or called may not subsequently raise an all-in bet that is less than the full size of the last bet or raise. (The half-the-size rule for reopening the betting is for limit poker only.)

http://www.pokercoach.us/RobsPkrRules4.htm

Although this is one of the rules, I think if this is a home game/tourney then you can make your own "house" rule as long as its applied consitently.