Here's what you can use variable materials ultimately no matter if there is wiki info on it or not:
1.
Variable materials wiki page comprehensively explains mechanics:
Primary/secondary ratio is 6:4
Primary/secondary/tertiary ratio is 5:3:2
2. In order to understand which material comes where, hover onto material icon in building window with material requirements.
Icon itself is split into as much zones, as there are materials type used for this material.
Hovering certain material over this icon zone with item in your mouse will display this material in the blueprint.
For example, here's me hovering laurel board over secondary board material on the table, resulting in showing me how secondary board material will be displayed after construction, which is door of this stonestead. Hovering over first part (separated by black line) will show how laurel board will be displayed as a first variable material respectively.
3. Use community tools, like Kerrigan's(known as an alpha male) hmdt:
https://carrygun.gitlab.io/hmdt/
Known as zunzon. Contact discord: zunzon.