My Bernina has a speed adjuster which I find very helpful. On my old singer I had to depend on the foot pedal. I had a hard time not pressing too hard on the foot pedal while moving my hands.
I depressed my foot pedal with my hand until I got the speed where I wanted it and marked it with chalk. I then used duct tape to stick a piece of wood to the foot pedal so I could only depress it until the wood stopper. It worked well until I upgraded my machine. If your machine is similar maybe this will help you.