Some embodiments provide a method for locking in a mathematical relationship between a set of at least two parameters in a computer program. The method selects a first parameter. The method defines a mathematical relationship that ties the value of the first parameter to the value of a second parameter. In some embodiments, the mathematical relationship can be a mathematical function, a Boolean function, or a lookup table. The relationship could be directly entered by the user, or the computer program itself could derive it from the user's actions. In some embodiments, the first parameter can be locked to the mathematical relationship. The lock prevents the value of that parameter from being changed, except by changing the value of the second parameter. Some embodiments implement this method by using a GUI with an intuitive set of controls.