The average line height of a font is 1.2 times the font size

Fonts have defined boundaries for glyphs but glyphs can draw outside the boundaries

Line Height means the distance between two corresponding positions in between two adjacent lines

Leading means the space between lines of text. Longer lines of text require more leading.

Descenders are the parts of a glyph that descend below the base line. If the bottom most edge of the letter "o" sits on the base line, the the parts of the letter "g", "j", "q" and "p" that are lower than the bottom of the letter "o" are the descenders.

Ascenders: if descenders are the parts of text that extend beyond the base line as with the example "o", above, the ascenders are the parts of letters that extend above the top most part of the letter "o". For example, "b", "d", "f" and "t".

X height/body height is the height of the main part of lower case text, exclusive of ascenders and descenders. X-height is so called because it is measured on the height of the lower case letter "x".

Cap height is the height from the baseline to the top of the ascenders.

Dyslexia

Justified text creates "white rivers" through the text because of the variable spacing. This can make the text very difficult to read. The effect can possibly be offset by using more leading.

Editing and Coverting Between Formats

There are lots of different formats of font files. What you usually have on your computer these days is not what a lot of code libraries are capable of working with. If I ever need to convert fonts to individual TrueType file formats I use FontForge. This is also available to install in MacPorts.