If you say 0.5 flights, it means they won't be returning to the homebase after that particular flight.
For example, if the airplane have 0.5 flights from KATL to KLAX, it means it won't be returning to KATL until the next day.
So think like this: if you want to fly from KLAX, you have to make a base there. Thus you can start flying from there to anywhere (as you pointed out that you want to fly to KSEA from there). Then, you can start KATL-KSEA to mimick thus what you said that the plane flies from KSEA-KATL. Or, if you want to really to fly from KSEA-KATL (taking passenger from KSEA, instead of KATL), then you have to start a base there too.
y Airline is: Star Air Indonesia (W4)
My Aim is: Creating a real-world like airline with reasonable flight frequencies and rational aircraft type of fleet, by not necessarily hogging the game's nature/bugs/lacking.