Yeah, I agree additive is generally better if you care about computation(though you can do fun stuff with taking only powers of 2 and then multiplying the ones you need if you want to get fancy on the other
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My issue is that stating that its a .95x cluster attack penalty doesn't really work in any sense really(unless its multiplicative--which it isn't). Arguably what you should say is that its a -.05 cluster bonus or a .05 cluster penalty if you want it to be consistent with the way autocannons work/are written.
Edit: Imo exponential would work for the penalties for two reasons both dealing with the fact that the ship caps are so tiny(first the computation isn't that bad if you are only raising to the 10th or so power and second that its going to get tiny and hit the cap pretty quickly).--However it would be really confusing to have the two mechanics work differently so I'd be in favor of keeping them both additive.
Edit: Basically its really confusing thinking that if you combine .95 and .95 you get .9, and if you combine .95, .95 and .95 you get .85, etc. Seems more intuitive to think that you combine .05 and .05 and you get .1.