Should be a set-up option to calibrate the plotter (ie. a plotter option). It is probably self contained within the plotter. As you say, the width is fine because that is contolled by a fixed encoder device (though often you can fine tune this too). The length is controlled by rotory encoders. The paper is driven through by rollers. Believe it or not even paper thickness and surface finish (friction!) will efect your plot scale along the long axis. Also the rollers wear and their diameter decreases (it don't take much!). BTW: You can buy replacement rollers. I am not familiar with the 750 model specifically, but it must have this calibration capability. Try looking in the plotter manual.
We have need to plot very accurate optical targets and other things from time to time, so we always go through a plotter calibration check first.
(Note that although the paper is "flat" going through the plotter, the upper surface speed/distance is dependent on the total effective radius to the paper surface - ie. the roller radius plus paper thickness.) The rollers are not very large, so even a few thousandths of an inch difference in thickness can make a significant change in the total plot length.
The usual process is it prints a cal plot that has two lines of a supposed length - one longwise, one crosswise. You measure them and enter back in the actual length. The plotter than adjusts and stores the new settings. It then plots a new cal plot and you ususally have to go around two or three times depending on how accurate you want it.
John Richards Sr. Mech. Engr.
Rockwell Collins Flight Dynamics
There are only 10 types of people in the world - those who understand binary and those who don't.