How does Drop Rate Potential stack with other forms of Drop Rate?

I've always been under the assumption that it just stacks multiplicatively with everything else (2x drop * 2x card * 1.2x Greed Pendant = 4.8x drop, for example), but is this correct? For some reason I feel like it could just be additive on top of your current drop rate (2x drop * 2x card + .2x Greed Pendant = 4.2x drop, for example). Is there any way to accurately test and confirm this?

I wasn't sure how to simplify the title but I wanted to make it as specific as possible so whatever, it works.