Using Willows (with and without)
Without 5th piece of Willows - Mag regen at 1665 (live)
With Willows 5th piece (TOOL-TIP - Increase ALL recovery in combat by 15%) - Mag regen is 1817 (live)
Should be 1914.75 according to the tool-tip.
I am aware that there was a change to additive from multi but this tool-tip is multiplicative in it's explanation. (15%) Total add from Willows 5th piece is 152 or 9.129129129-%, not 15%. The other regens are about the same percentages. I have no idea how the computation is being figured now.