The 790 like the 865 will vary per RPM and heat thinning the oil. I don't think there is a published spec. 5-15 psi at idle depending on the temp would be good. 35-55 psi in operation is probably a fair number under load. Over 65 I would say the pressure is getting bad and over 70 we should hope Triumph has a working relief valve
I really don't know, just typing out loud. So my advise is no pressure is very bad and pressure over 70 psi is very bad. Thats why I did not buy a gauge I would just worry and look at it. What I don't know makes me happier.