It depends on how the nongravitational parameters
are computed.
For example, without nongravitational parameter
rms=0.810",
using the MPC's nongravitational parameter we have
rms=0.640",
and with the JPL NASA's ngrv parameters we have
rms=0.534".
Computations were made using all 2311 observations
of comet C/2012X1 and the OrbFit software.
Best regards,
Ireneusz Wlodarczyk