This is an interesting subject for me (just starting NATs selection). One of the obvious considerations is Wake Vortex Categories.
The 747 is Heavy, A320 Medium, and Cessna Light. The information I'm reading states minimum separation when the preceeding a/c is in a higher category. So, if a Heavy preceeds a Medium then 5nm separation is required.
However, what happens when a Medium preceeds a Heavy? Where would I find this information?