I did some testing a little while ago on PTR, and concluded:
each point of Defence above 400 gives +0.15% additional (eg. incremental) IBF damage reduction.
I tested up to 596 defence and it was linear.
So at 600 you are 50% IBF, and 400 you're at baseline learnt 20%.
Test screenshots and so on here, and here. (Two posts within one thread).
Tarrke, your test results are consistent with my findings, so I think we can say it's proven fact now.
Only remaining question is whether it remains linear above 600 defence.