User blog comment:Emptylord/Archived/PBE Patch Notes (Preseason 3)/@comment-620927-20121201093406/@comment-50.148.105.185-20121203134836

Whythisname, your math is very off. They stack multiplicatively, and here's how

The new flash has a base CD of 300 seconds. If you have a 188 second CD with Distortion, Mastermind, and Summoner's Insight, that means

300*0.9 (Mastermind) = 270

270*0.75 (Distortion) = 202.5

202.5 - 15 (Summoner's Insight) = 187.5, which gets rounded to 188.

Sources of CDR for summoner spells stack multiplicatively, with % reductions calculated before flat reductions.