How does Seed Chip math work?

How does the math work on Seed Chips? I’ve got five of the basic grey background chips installed. Three of them have “Increases thermal weapon damage by X%”, plus one reads “Increases damage by Y%”.

The total Thermal damage increase adds up to nearly 200%, and the less specific “Increases damage” is 20%".

If I was doing 100 thermal damage without the chips, does that mean I’m now doing 320? That assumes they are additive; if the therms add and general is multiplicative, it would be 360. If they are all multipliers it would be 500!

So what is the actual deal here? I am quite surprised that I can get so much out of seed chips when everything else in the game comes in painfully small increments; it has me wondering if I have the whole thing wrong.