Hi together, because I like to clarify a bit about the system Aerofly is using and what AeroScenery is offering I made a video what levels getting displayed at what time:
Level 9 to 15:
So if you use AeroScenery (or any other type of generation) go for level 9 to level 14 for best resolution, Why? Because Level 9 to 14 uses for a level 9 size tile 2,7 GB data, if you add level 15 you end up at 9 GB. Use Level 15 only for small areas as an addidtion. (Using the same download level you use for 9 to 14.)
What does level 14 mean for the source? (Bing Map, Google Maps...)
AFS2 is using 2048 pixel wide tiles. So we need to find out, how big is the area we need to cover.
Let us take a tile at the equator:
Area
Name: 8000_8000
Level: 14
Ref Point:
Latitude: 0
Longitude: 0
BoundingBox: (minX;maxX;minY;maxY)
0;0,02197265625;0;0,0219726556569224
Height: 2,441 km
Top width: 2,441 km
Bottom width: 2,441 km
-> 2441 m / 2048 pix = 1,19 meter per pixel, horizontal as well as vertikal.
Therefore an Image Detail (Zoom Level) of 17 (1,194 meter / pixel) is nearly ok, possibly we need 18 (0,597 meter/pixel).
Lets go no 60° north: (Island Brae/South Greenland)
Area
Name: 8000_B600
Level: 14
Ref Point:
Latitude: 60,0007397481079
Longitude: 0
BoundingBox: (minX;maxX;minY;maxY)
0;0,02197265625;60,0007397481079;60,0119079020322
Height: 1,241 km
Top width: 1,22 km
Bottom width: 1,221 km
->
Height: 1241 m / 2048 pix = 0,60 meter per pixel
Width: 1221 m / 2048 pix = 0,59 meter per pixel
Therefore an Image Detail (Zoom Level) of 18 (0,597 meter/pixel) may be ok, possibly we like to have 19 (0,299 meter / pixel).
50° North (around Frankfort/Winnipeg)
Area
Name: 8000_A988
Level: 14
Ref Point:
Latitude: 50,0020713682087
Longitude: 0
BoundingBox: (minX;maxX;minY;maxY)
0;0,02197265625;50,0020713682087;50,0160462198495
Height: 1,553 km
Top width: 1,569 km
Bottom width: 1,569 km
->
Height: 1553 m / 2048 pix = 0,76 meter per pixel
Width: 1569 m / 2048 pix = 0,77 meter per pixel
Therefore an Image Detail (Zoom Level) of 18 (0,597 meter/pixel) may be ok, possibly we like to have 19 (0,299 meter / pixel).
And 40° North (Madrid/New York):
Area
Name: 8000_9F4C
Level: 14
Ref Point:
Latitude: 40,0017553835177
Longitude: 0
BoundingBox: (minX;maxX;minY;maxY)
0;0,02197265625;40,0017553835177;40,0183380933108
Height: 1,843 km
Top width: 1,87 km
Bottom width: 1,87 km
->
Height: 1843 m / 2048 pix = 0,90 meter per pixel
Width: 1870 m / 2048 pix = 0,91 meter per pixel
Therefore an Image Detail (Zoom Level) of 18 (0,597 meter/pixel) is ok.
Conclusion:
Go for Aerofly FS 2/4 Level 9 to 14 for best resolution.
For the input images from google/bing or whatever source:
Image Detail (Zoom Level) of 18 should make it most of the time.
So....
If you go further north or south from the equator you will need more source resolution.
Until 60° North or 60° South you are running ok with Source Level 18. Further North or South I would take Level 19. 70° or more possibly Level 20 but you will not find any source that far north or south giving this resolution.
If you want to go one notch up and have 2 pix in the source for 1 pixel in the destination you will be fine with Level 19 till 60° north or south. For > 60°N or <-60°S you will need 20 (good luck to find them.)
So my recommendation would be to produce Aerofly Level 9 to 14 is: Use Level 18 for the source until you are 55°North or 55°South. Further North or South try to get Level 19. Level 20 will be too much, for one thing the sources are often not so good as they should and especially they get downsized anyway. 
