The correct length for a kilt is to the centre of the kneecap. Not above the knee, not below the knee. This is the traditional Scottish standard and it exists for a reason — it is the length at which a kilt moves and drapes properly.
Kilts that are too short look wrong immediately. They sit up when you walk and the apron panels do not hang correctly. Kilts that are too long are impractical and restrict movement. The mid-kneecap measurement is not arbitrary — it is the result of centuries of people figuring out what works.