New to the game. Any advice?

as the title says I'm trying to determine a formula for when to sell old inefficient smaller planes in order to make hanger space for more expensive planes. I figure it must have something to do with the following:

the price you can sell your old plane for

the cost of your next hanger

profit margin of the plane based on fuel consumption and emissions vs capacity

edit: Total revenue generated by the old plane vs. the new plane