I recommend zero spread for basic cannon,with slight spread as cannon slave approaches 100%. (1 m spread at 250 m, maximum)
I know it seems less fun with less spread, but this isn't a hand-held weapon: it's a vehicle-mounted, aerospace-engineered precision system.
On the other hand, "spread" would be an interesting additional effect. Equivalent to "shotgunning", but instead of spreading multiple beams/shots at once, it would "spread" one shot at a time. You could use this effect to increase spread at will. Passive effect? Maybe increasing the spread of the weapons in a ship or turret that has you targeted, to simulate a refractive shielding effect that causes shots to slightly bend away from hitting you.