If you're deploying using the Scenarios, the enemy should never be more than 120cm away.
If you're not deploying using scenarios, the Nova Cannon only has a range of 150cm anyway, which is less than one extra turn of movement. You could play on a 10km board, and Nova Cannons would not be any more effective.
The same tactics as before should work:
#1. Go AAF as much as possible. Carriers start loaded, so can AAF as well.
#2. Screen your battle line with cheap escorts. If it takes three Nova Cannon shells to waste one cobra/equivalent, job done.
#3. Squadron your first 2-3 cruisers, and always move so that they're an equal distance from the enemy's fleet. This will dilute their fire and magnify the effect of multiple ships with shields.
#4. Allow any ship that's taken more than one or two hits past shields to drop back in formation. Better to allow another cruiser to take some damage than to risk a ship being crippled.
#5. Don't brace unless absolutely necessary - say that despite your squadroning and escort screen the opponent manages to focus 4 or more Nova Cannons on the same target.
Remember, the Nova Cannon is a weapon of terror. Occasionally it can sting you, but for the most part its bark is worse than its bite. The average damage from a Nova shot is in the region of 1.3 hits, and it has to get past shields. The average S6 torp volley will do 1.7 hits, past shields, plus any follow up attacks on subsequent targets, another 0.75 hits on a single target. Call it 0.35 if there's a 50% chance of hitting a second ship.
To match that one turn of damage, the Nova Cannon has to fire an average of three more times than the ship armed with torps. Suppose that the two fleets start out of range and the Imperial ships move up to 150cm first to fire. 1 shot. They get a 2nd shot at 101cm, and a 3rd shot at 52. There should be no fourth shot.
In a fleet with 7 Novas, that's 21 shots. The average leadership is 7.5, so of those 21 shots, just under 9 will shoot at the escort screen. The remaining 12, split between two squadroned ships over 3 turns, can expect an average of 3.3 hits past shields, divided between two cruisers. Of course, all that is assuming open terrain and an Imperial first turn. Even from 160cm starting distance, a chaos fleet still has a pretty good chance of closing too quickly for an Imperial 3rd shot.
The strength of the Nova Cannon is that if the enemy stays at range, it will eventually wear them down without ability to reply, so it forces them to come close into the teeth of torpedos, which will actually do more damage. Spamming Nova Cannons still forces the enemy to come to you, but leaves you almost entirely toothless when they do.