A lot. it's not like the table is of infinite size. On a 6x4, there is only so far you can go before the imperial phalanx corners you on one side of the table.
Another thing i would like to point out is that it seems like your trying to make IN ships be strong enough to function independently. The problem is, they simply were not designed to do this, most IN ships were meant to function in a squadron of 2 in order to be effective. From what i have been able to determine, the only IN ship that functions well on it's own is the Armageddon.
I will maintain my point, try the 2x overlord list I made. You may be surprised at how powerful it is against just about any opponent. I have taken it up against Chaos, Diasporex, Orks, Nids, eldar, tau and necrons and it always did a great job (while i did not always WIN, the enemy never left the field in one piece)
here is the basic tactic for the fleet.
Keep everything in a squad of two, aside from the gothic, which acts as a 'wild card'. keep the lunars together, keep the dictators together, keep the overlords together, base-to-base with their squad mates. Deploy the overlords about 30cm apart from the rest of the fleet, to give the impression of splitting forces.
While your moving towards the enemy launch bomber (or fighter squadrons if necessary), and torpedo attacks. While your standard cruisers engage the enemy, the Overlords should be moving around the side to flank the opponent, constantly firing their weapons (typically using batteries to knock down shields, lances to chip off hits). their extreme range will keep them (reasonably) safe. another, more risky tactic is to turn parallel with the table, and have them send their broadsides directly into the oncoming enemy ships.
If the enemies go for the overlords in large numbers, turn them in towards the center of the table, launch torpedo's and bring your fleet around to support, locking the enemy in a pincer attack of torpedo's and death.
Against eldar, keep the Overlords in the center of the fleet, moving the standard cuisers out first to form a perimeter around the overlords, giving them some protection as they fire at long range. if the enemy tries to attack the perimeter, they suffer from the overlords attacks, if they go after the overlords, they suffer from the perimeter's attacks. Use carriers as torpedo and bomber defense.
Against necrons, keep the Gothic close to the overlords, and AIM FOR THE TOMBSHIP! (shocker i know). bombers should be attacking scythes and shrowds. (yes the scythe has three turrets, but it's better then throwing them at a battleship or escorts.)
Against Nids, focus your fire on the hive ships while your torpedos and bombers attack the escort craft (especially those thrice damned krakens) IF one of your cruisers is boarded, don't be afraid to 'push the button' for a chance to cause a lot of pain to their ships before you open fire with the rest.
Against tau, all of your carriers should be focused on CAP and torpedo interception, move the fleet in as a wedge to break through the Tau phalanx and get behind their ships do not split your forces or you will find the enemy focusing on whichever side is more vulnerable (aka, the side with fewer carriers!), once you force them into a close brawl, the Tau suffer. Keep the overlords at a distance shooting at targets of opportunity, and aim for their big carriers. Ramming, boarding and close range combat are your best friends.