Since we are all at it, take a look at my "The Ultimate Magic Square", which combines the classic magic square effect with a visual climax. Also, since you are not writing the numbers down, but rather use 16 cards which you hold in your hands before the spectator names his number, it is even more magical. The effect is therefore not one of great mathematical skill, but more of "wow how could he know beforehand what number I will pick, since the numbers are printed on the cards". (The effect is done with 16 cards, so no sleight of hand necessary to achieve this effect.) http://www.lybrary.com/books/TUMS.html
preserving magic one book at a time.