Convert between color tint/shade and luminance in WordprocessingML and DrawingML

Question

I am looking for the mathematical relationship between color tint/shading and luminance modulation and offset, if there is any, or at least an approximation of it. I need to know this because i want to convert font colors in WordprocessingML such as:

<w:color w:val="FDE9D9" w:themeColor="accent6" w:themeTint="33"/>

to the corresponding colors in DrawingML such as:

<a:schemeClr val="accent6">
   <a:lumMod val="20000"/>
   <a:lumOff val="80000"/>
</a:schemeClr>

These 2 samples seem to be equivalent, they were applied the same way in MS Word, however it is not clear how you can determine one from the other and vice-versa.