I'm searching for the method to calibrate what I type into LXI in inches to actually cut that size. For instance, I cut a line of text and set the size at 60". However, the ACTUAL finished size was about 10" longer. Any suggestions will be greatly appreciated.
If you're off this much, you have something else going on. A 60" cut might be +-a fraction but not 10".
The way it works is...
The software, in your case LXi, knows how long, physically, a plotter increment is. Say it's .01mm, just as an example. The software wants to move, say, 60" in the X direction. The software calculates the number of plotter increments in 60", a very simple ratio calculation, and sends something like 'PR23622,0;' to the plotter. There being 23,622 plotter increments in 60".
How far the plotter actually moves depends on how accurate an precise the plotter's hardware. An error of 1 part in 6, as you've reported, is inconceivable. Even stone-age machine used by Thrag of the hill people would exhibit better accuracy than that.
Thus you have something else happening.