Think of a recipie. You need to include all the ingredients to end with say a cake, but for this particular cake ( say a nice easy victoria sponge cake loL!z!?!g`3!), not all the ingredients or processes have to be in order. Regardless, it is still a recipie that is used to create the cake evertime. And thats the closest thing your gunna get to a formula when describing bands, because obviously no band is going to say 'right, well, to make every song we are going to create, we have to do this cleanguitar/distortedguitar/back to cleanguitar/ etc etc'. But they will have certain guidlines that defines their style of writing, regardless of what order it is in. So in that sense Opeth do use (What we can describe) a 'formula' as the majority of the songs include these attributes (even if its not the true mathematic formula)