Actually, it's pretty sure that a sufficiently detailed recipe for any potion followed to the letter will get you the same potion of similar quality every single iteration, if only because otherwise potions is completely useless as a subject. In fact, in many ways chemistry is like that too, with part of the assumption being you start with clean tools and raw materials of sufficient quality (well, purity when you are dealing with chemicals). And as long as you follow the instructions properly, you will constantly and consequently get the same results. That's why you can do science to chemistry in the first place. (It's also why you can apply the scientific method to cooking, same ingredients of similar quality with similar gear provides similar results...)
The point of the potions class however isn't just to teach students how to follow recipes, although many will be satisfied with learning that and dropping the class after their OWLs, no part of the lessons is thinking critically about potions, their ingredients and their interactions, preparing students for research and development involving new potions or identifying potions from their effects or a (partial) ingredient list, but that would mostly be part of the NEWT level classes preparing a student for a Master in Potions.
The point of the potions class however isn't just to teach students how to follow recipes, although many will be satisfied with learning that and dropping the class after their OWLs, no part of the lessons is thinking critically about potions, their ingredients and their interactions, preparing students for research and development involving new potions or identifying potions from their effects or a (partial) ingredient list, but that would mostly be part of the NEWT level classes preparing a student for a Master in Potions.