That’s real similar to what my 68 400 HO’s oil pressure runs.
After mine was good and hot it would drop to about 10psi maybe even a hair less when idling around that 800-850 rpm, I adjusted the idle so it’s up around like 900-950 rpm and just that little extra keeps it up around 15psi. I’ve always run Driven HR 10w-30 and the last oil change I decided to try the Driven HR 10w-40 and I actually might switch back to the 10w-30. For the first 10-15 min with the 40w it actually gets up around like 75-80psi if you get the rpms up there, once it’s good and hot it settles in at about 60-65psi at cruising speed and about 20psi idling around that 900 rpm range.

I was always told an old stand by is 10psi for every 1000 rpm, if that’s the case 10psi should be plenty idling at 850 psi.🤷🏼‍♂️