You need to work out how much potency gain comes from 10 natural blood. You look at a natural blood spiller, how much potency you gain from that, divide it by 10 which is then added to souleater and divide by 3 for use per GCD.
By assuming 50 blood = 400 potentcy, you ignore the fact that blood spiller pushes your gcds forward by one each time so it is definitely not a raw 400 potency gain.