unicode-math | A UnicodeMath converter

I am creating a series of PDFs in an RMarkdown loop, and can't figure out how to control the spacing between the lines created with \headrule and the start of the text within the r chunk.

My attempts have been futile, but have included using \vspace{#pt} in various locations, adding cat("\\vspace{#pt}"), trying to use \usepackage{geometry} while specifying headsep. I'm at a loss and appreciate any and all advice/solutions.

I would like to move the text closer to the blue and red lines in the image below. Ideally the adjustment can be dynamic, since the general outline will be used as a template for similar but slightly different looking reports.

Here is a reprex for .rmd to pdf:

I'm working on a couple of latex templates that belong together (letter, article, report). I've achieved nearly all my objectives except for a single one, having the font size change using the \documentclass[.pt]{} command, whether I use 9pt or 12pt my document looks the same. How do I ensure that this works correctly for my custom class?

Just some sidenotes, I use XeLaTex to compile my documents while I install the fonts (DejaVu familiy) with the fontspec package and the \setromanfont{DejaVu Serif} command. The classes have the option to switch between the serif and the sans-serif font in the family.

Here is my minimal working example: Consisting of the letter.tex file.