What formula do you use?

Question

To calculate the proper cruising altitude for a flight

Answer 1

There is international laws governing the height that a plane should fly. There are different rules in place for aircraft separation in controlled airspace and outside controlled airspace. The direction that the airplane is travelling between the destinations will determine which heights you need to comply with. This will ensure that aircraft overflying a certain spot will be at different height when coming from different direction. For those interested these are the quadrantal rules and semi-circular rules.