According to
Astrometry.net, your f/number is f/5.32 (assuming the FL of our scopes are identical, which they are not.
This is consistent with ~125 mm backfocus..
C11
Size: 27 x 18 arcmin
Radius: 0.270 deg
Pixel scale: 0.312 arcsec/pixel
C11_FR 114 mm
Size: 48.5 x 32.3 arcmin
Radius: 0.486 deg
Pixel scale: 0.561 arcsec/pixel
Reduction: 0.55
F/5.5
C11_FR 103 mm
Size: 45.8 x 30.6 arcmin
Radius: 0.459 deg
Pixel scale: 0.531 arcsec/pixel
Reduction: 0.589
F/5.9
Meade 11, Celestron 6.3 FF/FR, backfocus 125 mm
Size: 50.8 x 33.8 arcmin
Radius: 0.508 deg
Pixel scale: 3.74 arcsec/pixel
reduction: 0.5315