First orbit# is at the MAO, second is MAO + Spread, third is MAO + 2 x spread, etc.
Keep in mind there can be empty orbits, and also Orbit#s less than 1.0 get divided by 10, so if your'e at 0.3 + spread of 2.1, you'd actually only add 0.3 + 0.21 to get 0.51.
I believe there was an option to...