Is it as simple as this?
Enter Nr & Tq, where they join, go up, follow the lines until you hit PA, then extend a line from there up as far as you like (line 1);
Then come in from the other direction with OAT across until it hits Ng, go straight down to the 'correct/incorrect' guiding line and then across to the left until you hit line 1;
if the intersection falls in the 'correct' part it's OK?
Don't know, but that seems like it could be what they're suggesting with the reversed lines.