Starbase Assist (technically ship assist) is an absolute value, not relative. A starbase with 50 attack will give the same bonuses as a starbase with 0 attack, provided the assist modules are installed.
Starship Quality increases the Attack stat by the stated percentage for ships with attack. Interestingly, if you build a ship at a planet with a Starship Quality bonus and later upgrade it to one with weapons, it also gets the bonus.
I remember ships built prior to the Stellar Forge also retroactively getting the bonus if they were built on the same planet, but I could be mistaken.
I don't remember how it interacts with weapons bonuses; whether it stacks or whether it multiplies.