At that distance and firing position, the enemy is showing it's smallest target profile and the wing root cannons tend to shoot under the Raider and the top cannon shoots over the Raider. The converging distance isn't surefire shot either because the shots cycle rather than fire simultaneously, and the dispersion spreads the rounds a bit too much to my tastes - actual 20 mm cannons are typically very accurate, they just have big recoil which means that it would be more realistic to configure them firing simultaneously which would produce an even recoil. I believe that the series depicts one out of three tracer round configuration which was also typical in WW2 fighter plane ammunition - not all round needs to be visible. I just don't know how feasible this would be to realize - meaning the ammunition loadout types, which would be awesome to configure yourself along with the convergence distance...
Having chance to put in two HE rounds, then one incendiary armour-piercing tracer round... with slightly differing damage/armourpiercing qualities. And convergence distances would be very good to have configurable by player, since people tend to have different flying styles. In IL-2, convergence distances of 100-200 metres are the most functional ones.
Practical help to hit the Cylons where it hurts: roll, and use your left/right/up/down maneuvering thrusters while following the Raider to aquire an advantageous firing position from it's upside or bottom, where it presents largest target area. If you manage to get into a position where the Raider is not moving very fast through the reticle, you can pretty much shoot straight at it.
Also, I personally turned the lead indicator off and use glide mode extensively in combat. The lead indicator isn't always exactly right and I can actually do a better job myself. Deflection shooting is a skill you just need to learn, though... playing IL-2 Sturmovik helps surprisingly lot. Basically to begin, take twice the amount of lead you initially think is necessary, then fire a short burst and observe where the rounds go. Learn to anticipate the Cylons' trajectories and your rounds' velocity.
Multiplayer is another ballgame entirely, and scoring hits with primary weapons requires extraordinarily good response time conditions or being the host... Secondaries are in much more important role than in single player (largely since the single player minicampaign didn't have missiles, but whatever
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