Yes good points, but I have 2 manuals from different eras with the same specs and the "5" stamped on the valve housing. Also applying the rule of thumb that's been around for 100 years and still holds true of approximately 10 psi/1000 rpm and with the high cam engines turning max rpm of 8400rpm then a 5 bar or 70psi relief valve sound about right.
Also there's no guarantee with a relief valve that the original spring hasn't sagged so measuring its crack pressure wont necessarily give you a baseline.
Ciao