Technical font terminology question

ginger2snaps's picture

Background
The high level problem I am trying to calculate the width of a line of text for a given font. I am generating a dynamic PDF using fpdf.org's tool. FPDF utilizes PHP files among other things to generate the PDF. I can use this PDF to generate the length of a line of text given the definition. I've provided two sample definitions below, one for Arial and one for Amazon (a script font).

Question
My problem is I want to make sure I'm understanding what all of these variables are first. I don't understand what the following variables are:
flags
ut
up
stemV
As you can see from the images below, Arial isn't really caluclating correctly, but Amazon is. The reason I don't think Arial is calculating correctly is because the ration between the EM Square =2692 and the EM Width (width of an U.C. "M") =833 be SO far apart. Am I misinterpretting the numbers and/or variables?

FOR DETAILS AND IMAGES SEE:
http://www.varigata.com/type/fontsample.html

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