I have basic knowledge (in general). But I still want opinion of other researchers and good sources to refer. TIA
Every XRD software produces a figure of merit which give the quality of the model with respect to the experiment. However, it is necessary never forget that software is only mathematics but physics is from author's account.
You can check the R square value, for confirming how good is your fit.
After the Rietveld refinement, you get residual factor (R-factor) in output file. These factors are based on different references but all are defining residual between experimental and calculated patterns. For good fit these values should be very low (less than 10-12%).
R factors and GOF are good indicator of high quality refinement. You can find more information in the following reference and references therein:
“Synthesis and characterization of pure and cobalt doped magnesium oxide nanoparticles: Insight from experimental and theoretical investigation”
Basically, R factors and chi2 are used to evaluate the quality of a Rietveld fit. Generally, a low value of these agreement indices indicates a good quality fit. But, it is hard to tell an R value below which the fit can be treated as good. Because it depends on several other factors like, quality of the data, complexity of the structure etc.... You may go through the following literature to know the meaning of each agreement indices and what is treated as a bad fit.
https://www.cambridge.org/core/journals/powder-diffraction/article/r-factors-in-rietveld-analysis-how-good-is-good-enough/17439A1F889B689C495549A234D53682
As Prof P. Grima Gallardo suggested, a better physics may come from a large value of R factors also. So it is better to look for the physical meaning of the refined parametrs rather than too much worrying about the R factors.