I did Sable,
My results were I had the most stability with the ARCA mounted on the two swivel studs. Then I mounted the ARCA just forward of the front receiver screw. The reticle was kind of drifty. Not good.
To my surprise, near the receiver screw gave me the best groups. Even though its more reticle drift. Out near the end of the stock where it was more stable they were the worst. With plenty of vertical. This was tested at 717 yards on three different days with the same results. The stock is a A-5 McMillan.
The rear of the rifle is supported with my elbow resting on a sandbag on my thigh. I sit in a chair . I have very good stability in that position. I'm testing this Rifle to replicate in the field hunting positions. If I have time to do more testing I'm going to try to determine why I'm getting more vertical with the ARCA on the end of the stock forearm yet I have less reticle movement in that position. I lean on a rifle pretty hard to control the recoil of the Rifle. I may be inducing flex in the stock with the ARCA in that position. We'll see.