I'm not sure how the FLAT would work. But, when I copied my char to the beta server. And using the magic of popmenu to create the needed IOs. I was able to see the effects on the combat attributes screen.
I unslotted all IOs that gave a regen bonus, then I unslotted all IOs in health. My combat attributes for Regeneration was this:
Then, I slotted the lvl 50 IO. I saw that my health went from 17% to 24%, which is a 42% increase on the 17%.
Lastly, I unslotted the lvl 50 IO and slotted the numina IO. I saw a new line called numina come up with an additional 8%. Looking at the base of 8.28 hp/sec and multiplying that by 20%, gives me 1.66 hp/sec. So, it looks like it's 20% of the base regen.