downlink budget

When considering the antenna size one can usually look up the EIRP (for the receiving site) and correlate that figure to a nominal sized antenna (using one of the many charts available).
My question; How can I be more specific with the antenna size required based on the modcods, and if I'm using the specs off Lyngsat which state typical C/N values (depending on the transponder) how can I calculate the MER value when using a simple signal analyzer meter to see the carrier (most will have this measurement in-built)?