Control Panel\All Control Panel Items\Power Options
Select your used power plan -> advanced power options (near the bottom).
The maximum processor state can be set lower than 100% in this case.
This means the closest clock multiplier will be used for that percentage.
Example following the screenshot:
If you have a, let's say, 3.00GHz processor @100%
--> for 85% => 3.00 * 0.85 = 2.55 and if the nearest possible value on that CPU is 2.4 or 2.6GHz, that value will be used.
Give it a try, testing the values for your needs.
Don't set the maximum state lower than the minimum state;
More info on CPU power options is found here:
Power Options - Add or Remove