This page provides figures that may be used in the link budget calculator
If you are using a high power spectral density uplink such as a feeder link for
broadcasting or a VSAT outlink destined for small antennas, and have the
satellite transponder set to low gain, then put a figure like 30 dB here.
If you are transmitting a low power spectral density uplink like a VSAT transmit signal destined for a large hub antenna, and have the satellite gain set to high, then put a figure like 20 dB. You are vulnerable to uplink interference from other uplink antennas pointed to adjacent satellites and from cross polar interference from co-frequency users on your satellite.
If the transponder is filled with 3 or more carriers, you will suffer from transponder intermodulation noise. The level of intermodulation noise depends on the transponder operating point. For a typical multi-carrier transponder the input back off will be 6 dB and the output back off will be 3.5 dB. The C/intermodulation ratio will be around 21 dB. Put 21 dB in the box.
Modern satellite transponders incorporate linearisers which permit the amplifiers to be operated closer to saturation while keeping the C/IM ratio acceptable. This also applies to modern earth station high power amplifiers and BUCs which are intended for multi-carrier operation. Linearisers also benefit the transmission of higher order modulation methods like 8QAM and 16APSK where low distortion is important.
If you are operating with 1 or 2 carriers the intermodulation falls out of band and is negligible. Put in a number like 50 dB or higher, which is equivalent to virtually nothing.
If you are using a large receive earth station put a figure like 30 dB here.
If you are using a medium size receive earth station put in 25 dB
If you are using a very small receive earth station put in 20 dB.
Small antennas have poor off axis discrimination and receive more interference from adjacent satellites. Their cross polar alignment adjustment is also often poor.
In all the above cases, if you know the proper figure, put it in of course. The figures above are intended as a good starting point.
Page amended 27th Feb 2016