Calculating height of a scalable font when only given width

When giving font 0 a width but no height, the height should be calculated based on the standard font matrix. For font 0, that would be h:15 w:12. So for width of 65, the height should be calculated to be 81 based on that ratio, but when printing A0N,81,65 it prints way bigger than it does with just the width specified. How is the height calculated based on that font matrix exactly? In my case, we are getting a zpl string from a client and doing some calculations on it. Unfortunately, only the width is ever supplied.