Run at least 10000 iterations
Go to -> Options -> Reforge Plots
Start with reforge amount 5000, step amount 500 (ammount/step should not be higher than 20 or so or the graph won't show)
Check "Plot reforge options for haste rating"
Check "Plot reforge options for mastery rating"
You get something like this (this was 2000 amount/200 step)
0 in the middle is your current haste/mastery rating. Everything under the horizontal line is a DPS loss everything over it is a gain. Now just look where the highest peak is. It's at 1200 mastery to haste. So change gems, reforge, etc. to reduce your mastery by 1200 and add haste instead.
Rerun and narrow the amount/step values and adjust gear setup until you get something like this.