No, I'm pretty sure it works like this.

Reasonably presumed Wyvern max HP (for example purposes): 1200
Wyvern HP at 1%: 12
Formula under Steady Wing: 20% * (Max wHP + wHP lost)
Actual numbers: 0.2 * (1200 + (1200-12)) = 0.2 * 2388 = ~478

That's...quite nice actually, particularly on top of the other 40%.

Also, question. The ability description says absorbs, but your details say nullified. Which is it?