How do I make the form text spacing match the printer text spacing?

I've developed an application that allows the user to enter multiple spaces between the left and right strings, In this example... let's say that the left string is a product; the right string the price.. using multiple rows. The problem is this:

The distance between the left and right strings of the printer and form is significantly different. If the distance between the the left string and right string on a particular row on the form is 3 inches, it may come out to be 2.5 inches on the printer form.

The printer font and form are verifibly the same. Where am I making the mistake?