In previous posts I have commented that for my personal sourdough I use 100% freshly milled organic whole-wheat.(13% protein)
Like a lot of other home bakers I enjoy baking for family and friends, however there are those that do not share my preference.
To cater for this I run my flour through a sieve ( I have a 40 and a 60).
This is a hit and miss procedure as I am unsure as to the alteration to the protein content after sifting.
My hope is that someone is aware of this analysis being conducted either on this site or elsewhere
That (protein content) depends not so much on sifting as on how uniform and reproducible your milling technique. I guess you could weigh your flour before sifting, and te resulting fractions - after, and keep a record of it. I mean if you can live with "unknown but similar".
Most of what you are taking out is bran. Here is its nutrient profile: https://www.nutritionvalue.org/Wheat_bran%2C_unprocessed_57601100_nutritional_value.html
You will see that bran actually has a higher percent protein than endosperm. However, it's not gluten-forming protein.