multiplicative:
so if you have 2 pieces of 50% fire reduction gear it'll be like, first gear reduces by 50%, 2nd gear reduces what is left by 50%, so it is a 75% reduction.

1-((1-0.5)*0.5)=75%

where as 50%+50% = 100% reduction.

this way the upper limit need not to be hard-capped, as the cost will greatly out-weigh the benefits.