This measurment is purely an approximation. It calculates the letter height chosen, adds an additional percentage for letters that drop below the baseline, then doubles for every line added to your quote. For fonts with unusually tall lowercase letters as well as the fonts in the "Special" tab, this measurement will not be correct.
If you have a total height restriction, please let us know in the comments section. We will make sure it stays within the boundaries you give us. If we have a challenge making your quote stay within your guidelines, we will contact you.