[Ragnarok]

It defaults at 25%, unless you change the settings.txt line. Then you can change it with racial points or those crystal building things. So, supposing I set my race to +5 on the maint. scale, does that make maintenace 20%? Or something more complex? Same with the buildings?

I never have fully understood it either. But from what I gather set your race to a +5 then yes the maint. goes down to 20%. And so on up the scale. Up to I believe the max is +20 which will make you pay 5% maint.

Ok, next, there's the ability you can add that reduces/increases maintenance. Bases for example typically have a -50%. Now what?

I would recommend checking out the Monters from the DevnullMod. That is how they make their resources. They actually make money on maint. instead of losing money. This is done because they do not colonize planets and so they need a way to support themselves.