Hi Tim,

I am a holistic health and movement counselor who recently got back into playing guitar after many years of not playing. While from a musical point of view that may be a fine way of sitting, it is a postural disaster as it forces your pelvis forward, creates lower back pressure and overall messes with your spinal and pelvic alignment. It may lead to back or knee pain if done long-term. Here is the link to a study that was done in Korea on this.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4242947/

Cheers and good luck with your guitar studies, however you sit!