It all relies on daily pax and having hubs at both ends.
According to the Emperor, the max potential blue pax is 0.02% of your systemwide daily pax. My experimental Empire spamline really came to life at 4 million daily pax.
0.2%. The placement of that decimal point is crucial. Note that the total pax per route page and per airline rankings are different: the latter is twice the former.
So you can add 0.2% percent of your daily pax per rankings table, but that will increase your rankings-table pax by 0.4%. Opening ~150 routes should double your pax if you do it right (1/250 = 0.4% but you'll want to anticipate traffic growth).
The increase in prices tracks (1) default price times (2) an X factor based on total alliance traffic. That X factor doubles at approximately Logarithm base 10 of your traffic level.
Knowing these parameters, you'll have to make tradeoffs between (1) maximizing traffic, (2) maximizing individual route profits, (3) searching for routes with the highest default prices, (4) crowding high-default-price routes versus opening more routes at lower density but for a higher X factor over a lower default price.
Applying above all of these parameters is a meta-level about time and cognitive load efficiency versus game resource efficiency (i.e. how many planes and the size of your country/number of max routes).
EDIT: By finding routes with highest default prices, I mean per-mile-flown, not absolute prices. Even more specifically, it's not per-mile-flown, but the best tradeoff between per-mile-flown and miles-flown-per-aircraft. That maximizing price point occurs at much shorter distances than is commonly flown.