As already mentioned before, the top right point is the intersection of OAT and Ng. From this point you go either down or up to the line dividing the correct/incorrect sections. Then, you go either left or right to intersect the line coming up from the Hp chart. Point P does not signify any particular Ng value, it only signifies either "correct" or "incorrect".
Maybe it's also helpful to construct an "incorrect" example. Take the values from the original example and use Hp=3.000ft instead. The OAT/Ng intersection would be the same, so you go right and down to the correct/incorrect line the same way. But to intersect now with the line coming up from the Hp diagram, you have to go right into the "incorrect" section, meaning you pulled too much power for that altitude.