I'm with
Standard Noise on this. If the aircraft are getting a RIS they get information not separation. If the information includes a level based on unverified Mode C then that's what I'll tell the pilot.
I get the impression that you are saying that you will accept a level report based on any old setting and make a mental calculation to decide if it is within tolerance. Very professional - what conversion are you using, do you correct it for temperature and pressure, who has authorised that procedure, and what chance is there of you making an error if the Mode C is out of tolerance and you are busy?
Conversion factors, temperature and pressure corrections?? Get real. It's to do with providing the service that I have agreed with the pilot, not overcontrolling and separating every VFR pilot and his dog. And, by the way, just as you say, I take a pride in doing it very professionally.