What is the best source of information to get specific infromation on a plant-wide safty and relief device testing program ?

I am aware that OSHA has some general "vague" requirements (OSHA 1910) that a plant owner must be able to show records of testing. But what document puts these devices in different categories of importance ?

This issue rears it's head every couple of years.  In my experience in offshore south-east Asia, the governments used to allow the majors such as Shell and Esso to self-regulate their inspection and testing programmes.  With regular shutdowns and loss of production, management were regularly revisiting the ctiteria for vessel internal inspection frequencies and RV recalibrating frequency.  The general reaction to these rquests was to look at what eothers do, find that you do that same, and change nothing!
Sorry if this isn't very helpful, but seriously I don't know of any guidelines other than high level "safe-guarding consideration" statements, but I guess the old criticality, service, temperature considerations would be good "first principals" starting points.  I don't think you will get much joy from valve manufacturers on valve reliabiliy.
Regards.

The 10% overpressure is calculated with respect to PSV set pressure and is for spring operated valves only. The 110% is for single service safety valves. Please note that if your relief case is "fire" then max allow overpressure is 21%.

The meaning of 10% allowable overpressure is that the max capacity is acchived at 10% higher pressure than SP, but the valve will actually start to open at SP (and "shimmer" somewhat before that).

A pilot operated valve achives full capacity as soon as the SP is reached.

This is a very interesting subject, not only from the perspective of maintaining and testing relief devices, but for looking at your whole program.  What about the pressure vessels these devices could protect ?  Do you inspect these?  Is your relief device properly sized?  How do you know? Have you investigated all the senarios? Another good source for starting a program would be the NBIC (National Board Inspection Code).  Is your state governed by ASME?  Are you a code state?
From my experience a comprehensive program could take a long time to establish.  We size each relief device for the worst case, check set pressure before installation on a test stand, pull the device after one year, pop it and look at the as found condition.  At this point we determine based on valves in like service and in the "as found" condition if the inspection interval should be changed.  We keep a valve history on each position and have a maximum length the valve can stay in before it is pulled.  Occassionally we will a valve that has an exception (pops high, dirty, etc.).  This position now requires an investigation to determine why?  Another area we look at with relief devices that is often overlooked is to check sizing when the process changes.  A process change could change the relieving requirement! We also do external and internal visuals on our pressure vessels, and do some pressure testing as well.

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