Why did Marcellus Pye need Septimus Heap as an apprentice?

Macellus Pye was an Alchemist, therefore the number "7" was a big deal for him. Septimus Heap is the 7th son of a 7th son, he was able to tell when the potion Macellus was making was finished. Only a 7th of 7th would be able to tell. It is in Physik.