The square root "rule" says that sales increases with the square root of advertising. So if you'd double your advertising spend (2x) your sales would increase with √2 (1.41). Using that rule and your inputs on the left, this calculator will calculate the profit-maximizing advertising spend.
Of course this model isn't correct, but it has it strengths and can help you get a first feel of the direction to test. You should always test carefully before making significant changes to your ad spend. Read all about it in this corresponding article on LinkedIn.