User blog comment:Emptylord/Archived/PBE Patch Notes (Preseason 3)/@comment-620927-20121201093406/@comment-5490997-20121201111743

It looks to me like it's additive, just like with regular CDR.

I tested it with Flash, using the Insight and Mastermind masteries + the new Flash CDR boot enchant (Distortion).

All those combined resulted in a 188 second CD for Flash. The calculation that I found that comes closest to that is: (original CD - flat reduction)*(100-CDR%).

We get 15 seconds of flat reduction from Insight and a total of 35% from Mastermind + Distortion. The base CDR of Flash is 300 seconds, so what we get in the case of Flash and all those masteries/items: (300 - 15) * (100 - 35) = 185.25

TL:DR; Mastermind and Eleisa's stack additively. Distortion and Mastermind do and there is a very big chance both use a similar calculation (there are no reasons why it would be different at least).