Haste for MM is more of a “feel” than a “certain value is required”.
The higher the amount the more damage you lose, the lower the amount the longer you need to stand still and possibly lose aimed shots by needing to move.
I’ve tried it at both ends, I went high when 4set was obtained as it pushes a large amount of damage into aimed shot which = equals more time standing still. The mindset was more aimed shots, the more damage but it actually went the other way due to the crit/mastery lost. Low is the way to go.
Low haste means you need to plan well to “buy time” to get the aimed shots off, but it pays off as its much more damage. If you’re new to MM then a little more haste will help, but if you’re experienced then something around 6-8% is win.