13-16mm is the range for a 500-class. I'd start at 13 and see if the geometry works out. You should be able to get 12* collective with a pitch range setting <80 on the gyro. If you need 85 or more, go to 16mm.
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CopterX 450 Black Angel FBL "Night Rig" | ZYX v4.0
PRÔTOS 500 FBL "Sky Whip" | Robird G31
Audacity P6 FBL "Phoenix" | MSH Brain | 12s
Goblin Speed | iKon
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