format the results of a formula

Currently I can use conditional format when values are more or less than 0. What I would like to do is be able to conditional format based upon the result of a mathematical condition. For example. My target is 100 for each month. In Jan I get 90, this should be highlighted red as the expression (90100) is true. This is a way to show when actual values miss or beat target values without having to create a "difference" field that takes up more room on the report just to get a + or - value to conditionally format.