So, let's just imagine this :
You have a threshold at 700, and a rate at 100. Every move adds 100... in 7 moves you reach 700.
Now, a rate at 200, still for a threshold at 700. In 4 moves you reach 700.
Now let's be crazy... let's just not change the rate, and change the threshold to 400 and keep the original rate... you reach the threshold in... *drumroll*... 4 moves.
I don't know what advantage you'd get of having a configurable rate in addition to a threshold... besides making the code more complexe without any big difference and complicate the end user interface for more casual users. That said, if you give me more infos about your goal and what you think it can improve... maybe it'll change my mind. But just like this, I don't see the inherent benefit of adding this.