Most claim they want to set the 303's out farther to "scare" planes that are attempting to seperate. Now this therory i totaly disagree with, that tactic is for killing/scaring rookies. Rookies are the least of your worries, if you get geared up and in the mind set to kill newbies, veterns will eat you alive.. they are the real threat to you.
Set your cannons a tad bit farther than your Mg's, they drop a little more (cannons). As a rule of thumb i set my cannons atleast 25 yards farther than my MG's.
The most effective convergence is the distance you are comfortable firing at. If you are not skilled at long range shooting (400+ is long range shooting for a cannon bird) set it closer because you want the most power where you'll be landing hits, not where you would LIKE to be landing hits. As you improve in gunnery, adjust convergence accordingly.
I set up for a gun solution around 600-500, so i set my out far. But in my case i normaly fly something armed with only 50 cals and they are deadly at that range. For cannons i recomend 325-350, and as previously stated 303's around 300 (since indeed they have no stopping power past that, very little even at that range).
good luck!