According to Flysat's equivalence table, it might be ok (For Ku Band, not C Band).
_http://www.flysat.com/dish.php
BUT, not only would that be marginal even if whatever source you have for assuming a 40dBW Footprint Contour is reliable and accurate, but you need to consider that Footprint Contours are notoriously inaccurate.
Always best to take a pessimistic view and go larger, except if you have local robust evidence to the contrary.
As you will note from the Table, a 1 or 2 dB difference between forecast and reality makes a BIG difference.