The calculations are made easier by the fact that the EIRP of a satellite is fixed in power level and is provided by satellite manufacturers at given points over the earth. Required dish sizes have dropped because the new generation of satellites has consistently higher EIRPs. I will spare you the math for the downlink path because the uplink section had more than enough. The only safety net in satellite communications is a bigger dish. A dB here, A dB there, use conservative figures.
Chances are most results are good.
Pick carefully whom you have do them. My recommendation, call your space segment provider.