Resistances are % based, of course they are linear... I can't even at this point. Ok, look, what is 10% of 100? 10% of 200? 10% of 300? Plot those points, is it a straight line? Now do it again with 20%. Is it still a straight line?
Also, I thought you meant adding another SB, obviously. I'm not even sure what SCBs have to do with anything, no-one but no-one uses Prismatic Shields on combat ships and I seriously doubt that's what the OP got them for. What you are doing is called "moving the goalposts" in an attempt to change the argument. No thanks.