Palsch, sorry I can't give you anything but word of mouth - but I assume that the people who told me had their facts straight about it being additive, not multiplicative.
I just ran some tests and it certainly looks additive. Only tested with self damage from rocket splash and C4, but both showed the combined resist + flack being shockingly better combined than I would have expected. If people are interested I could try to upload a video of the tests, but it's pretty poor quality.
A rocket fired point blank at a wall exactly removes my full shield.
With flack
or resist it drops you to ~8/20 bars of shield;
- definitely one tick under for flack 4 as expected
- maybe
slightly over for resist but certainly doesn't seem to give the expected 45% resist though).
With both you are left with around 16 bars, or ~80% life.
IIRC (no video of this yet, but I could easily film one) firing all five rockets in sequence - without time for my shields to refill but putting up resist for each shot - left me with no shields and down 1 bar of health, suggesting each rocket was doing ~21% base damage.
Even if resist is giving the suggested 40% explosive resistance (at least on self splash) instead of 45%, you should still be at 90% resistance with flack 5 and resist shields. That's a lot of splash damage you could shrug off.
Fakedit: I went looking for
this spreadsheet to confirm what resist shield was supposed to offer and turn out it is only 40% resistance to all damage anyway. So these numbers look right.
The C4 tests gave less useful numbers because I don't have a base damage rating to compare to the resistances, but comparing the damage from just flack 4 to flack 4 + resist showed a reduction of just over 10 health/shield bars, or a little over two thirds of the damage done rather than just reducing the damage done after flack by a further 40%, so it confirms that the two are not multiplicative.