What’s the effect of having more than one scrap recovery arm?

After reading Hackworth’s table, I decided to test a bit. I got 2 scrap recovery arms, and started writing down every time I received some scrap. I wrote down both the amount I was promised and the actual amount I received. Then I put it in Excel, and compared it to 3 lists corresponding to Hackworth’s A, B and C (additive, multiplicative and independently rounded, respectively). It turns out the formula (at least for 2 arms, I assume it’s the same for 3) is:
Scrap received = floor(Scrap promised * (1 + 10% / (number of arms) )
or:
Sr = floor(Sp*(1+0,1*n))

Documentation:
Excel sheet with data
(I also have screenshots from 3 different amounts, but as a new user I’m not allowed to post more than 2 links in one post)

tl;dr: The arms stack additively rounded down