The link above is for masthead rigged boats and not really appropriate for you. The 311 is 9/10s fractional and will NOT have a masttop sheave or block. As you guessed, there is a sheave just above the forestay. Use this one, the loads here are supported by the hounds and is much more secure.
Some fractional Beneteaus (Like the First 36.7) run masthead kites by using a masthead halyard, but the boats were designed like that.