I do a bunch of runs on the mill and the way to calibrate it is measure the length of the belt. you can do that by marking it with a chalk line- then keep moving the belt as you measure it until the chalk line has done a full revolution. once you have the belt measured.
Mark it again at one point or use the same mark and and get someone to count the revolutions while you are running at a set pace. Multiply the revs in one minute by the length of the belt to see how “far” you have traveled in a minute and calculate the pace—- then see if it matches what the treadmill says.
Let me know how it goes!