This feature is available from version 8.3.6 and up
Now it is possible to add a surcharge factor, either on a work day or an off day.
Under employees you choose employee settings. There you choose Capacity & flex and then flex time surcharge.
In here you can change your flex time surcharge factor. You can set after a given set of hours the flex time surcharge is going to take effect.
The over norm hours indicate when the surcharge takes effect. If you set it to 2 hours then the surcharge will first have effect on the time entry after the 2 hours over norm.
The factor is the value with which you multiply your flex hours with. When you type 1,2 then the factor is 20%.