Hello,
I actually mix in a small room like this (little bigger) and I found better to have the monitor direction like in the diagram posted by Mack Caster. Don't know if I respect any of 38% but it work good like this.
For Windows reflection at my right side, I probably have near the same matter with a glued and very polished wood cupboard doors at my left side.
But I have a real and unregular wooden cathedral ceiling and maybe this give another diffusion, maybe more cool for my ear... I don't know.
A thing I would do is buy Hofa curtain but waiting for the right item on sale.
Know I have to move house so I wait for what I will found in the new one.
A thing read here and important too me is the use of phone for mix, and monitors only for check.
Sometimes I do this way. It work good, but stereo always need correction while phone give an easelier listning of everything (not necessary where things was, but just and first ear them).
Sometimes I do the opposite way. It work good too, my monitor give a real good stereo location of each sound and probably better frequency management than my phone. But my untreated room make my ear weary faster so I the more I work on a mix the same day, the less I am sure about my frequency choice.
Compromises is actually a real rule too me. I experiment with each and other way to do, and what I choose know is to follow of my mood depending what I had to work on... the pleasure to follow my mind and change it on my only request is actually the better way I found to work good and keep pleasure to do it.
Home that this way will change to somethings better the day i can have a real mixing room, not necessary bigger or full of good stuff. But with acoustic treatments just make my ear not weary too fast
See you
seb