Dusty one thing that might help with the control issue is this.
At the top of the treadle bar, up near the motor, there should be a wingnut that pre-loads a spring. If you loosen that spring the motor should engage with less sensitivity, and more control.
I don't know if all machines have that, but the few I have owned do and should help you out.
Also, to get uniform widths and stitches, use the foot as a guide. WHat do you mean mark? Well, since you asked, when doing a wide frech seam, (seen on most american and late model cars) I place the inside of hte foot right on the seam. And keep it there as you sew. On narrower ones (seen in higher end cars) I move the seam to the outside of the inner foot and keep it there. it's only about an 1/8th in. difference but the visual is huge.
Hope the makes sense.