OK I've done a bit of fiddling with how this lightning system works, cut down some long-winded and redundant code, and shifted it much closer to how I want it to work. My version will have the classic "Shift some DPS to the main gun", so 100 each for the side cannons from 150, and the main gun to 200 DPS.
When there are available chain targets, 50% of the damage is diverted to the chain effect, then split among the number of chains. If there are no chain targets in range, the gun simply does all 200 DPS (800 Damage blast) to a single target. So you can have, randomly, 800 damage spread between 1, 2, 3, or 4 targets, with a minimum of 400 damage ever going to the primary target.
Once Resin gives me the OK, I'll put it up on the vault for everyone as V0.2