Damage Reduction, How does it work?

In terms of calculations, does a 20% damage reduction mean a character only takes 80% of the damage, or is it similar to borderlands where there’s the diminishing returns (1/[1+resistance%]) which would lead to an 83.333% damage taken?

The general consensus is that it does not (edit: it does not have diminishing returns).

Did you try using the forum search feature?
https://forums.gearboxsoftware.com/search?q=damage%20reduction

I believe for most sources a dev on Reddit has confirmed that gear and helix benefits are multiplicative rather than additive. So it would be 1/(1.8x1.12) for example.