How do damage percentages stack up in the game?

The percentages add up in this game.
45% bullet damage + 55% bullet damage
= 100% bullet damage.
But this does not mean that the damage will double because there are other bonuses and debuffs.
The formula takes into account your base + coefficient + bonuses from weapons + bonuses from skills + bonuses from robots + % of the final damage of the ring.
You can try to calculate how elemental damage is calculated yourself.

And in any case there is a coefficient from bm. Because a Persian with 50 ml bm deals damage disproportionately more than 25 million bm. Not 2 times, but 10 times more.
Damage grows non-linearly BM