SatmasterPro MK8.X Csat/XPI

Does this help: https://www.satsig.net/axial.htm ?

The relative levels (eirp down of all signals in the relevent bandwidth) of the co-pol and cross-pol carriers are  important. If the co-pol (wanted) signal power spectral density is low level from the satellite and the cross-pol interferer is high level then you need extra good x-pol isolation.

The transponder bandwidth, input and output back off are of no relevance.  It might happen however that the cross-pol transponder is operated with a single giant carrier at -0.5 dB output back off while the wanted co-pol is operated -in multicarrier mode, backed off by -3.5 dB, in which case you need extra good cross pol isolation.

When you do the link budget the overall C/N is basically the addition of all the components:
C/N = C/Nup + C/Iup +C/intermod + C/N down +C/I down.

This will add them up for you: https://www.satsig.net/linkbugt.htm

Try to improve your C/I down. 17.7 dB is awful.  30 dB excellent.  

Some cheap receive-only C band circular polariation feeds had 17 dB XPI in the FM TV days when interleaving of the co-pol and cross-pol FM carrier centre frequencies reduced the interference effect. Unfortunately with digital carriers interleaving does not help and it is the total of the interferers in the receive bandwidth that matters.

Best regards, Eric