Its not to clear what you are trying to do. The idea you called stupid is the percent difference formula, which is what you want to use for getting the percent difference. so the percent difference of 3 and 4 is ~28.6%.
Or do you mean in general talking where people say something like 'we...
When I posted this, the two previous posts where someone selling research chemicals, "We sell research chemicals". If anyone is reading this and curious about the title, it was a joke about those posts, not a non-subtle attempt of selling drugs on a physics forum (who does that?)