When set up that way, wont it give a blanket 5% increase to all planets? I'm going for adding it only to the planet being affected by Assimilated Populace. My goal is to have the buff that AP applies to a planet include a permanent 5% boost to allegiance. Which is also why I was asking about the base value %. This may be impossible the way I've outlined it, I may need a completely new buff or something. Just wanted to see if I could just tack it on. Thanks for the help.
No, there's actually no way to give a temporary max boost to allegiance or to specific planets, it has to be global. Base value is the value you get without having to research it. So if you did this.
All planets would have a +5% max allegiance from the start of the game, and once you research the tech you'd get an additional +5% per level.
I personally am not aware of any technology that actually does this, but you have the option too via a mod...
The base value line is by default used by the first fleet supply and capital ship techs, otherwise you'd have no fleet supply or cap slots available. Also the Advent Loyalist tier 8 tech that gives damage in culture bonus has a +5% base value, so they actually have that bonus without doing any research.