[Suicide Junkie]

Why did he do that, anyway? Is that how he "fixed" the maintenance aptitude trait?

Correct.

I set the basic maintenance rate to 100% of ship cost per turn.
I then added a 75% maintenance reduction to the ship hulls.

Since the maintenance reduction trait SUBTRACTS from the basic maintenance, a race that takes +20% to maintenance reduction would pay 80% maintenance per turn. The ship hull's ability then reduces that to 80% x (100%-75%) = 20%
A race that takes +60% to maint redux, will end up paying 10% maintenance instead of the normal 25%.

1) Race maintenace modifiers and the basic rate will ADD with each other.

2) Ship/base/component modifiers will ADD with each other.

3) The two Groups' totals are multiplied together for the final calculation.