I've found that the needle is normally well into the green on single-engined aircraft at around 1500rpm.
I agree that congested inlet filters are usually the cause of low pressure. I once saw one that hadn't been changed, IIRC, for seven years according to the installation label and the regulator garter filter was brown and literally crumbling. The hoses weren't in much better condition.
System leaks or a fading vacuum pump can obviously be another reason for low pressure and are sometimes masked by adjusting the regulator without properly looking into the cause.