AN MLG PLT QUESTIONED AN ALT ASSIGNMENT.

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WHILE CRUISING AT FL180; I WAS TOLD TO DSND TO 17000 FT WITH THE ALTIMETER AT ORD OF 30.48. I SET 30.48 IN MY ALTIMETER AND NOTICED THAT IT MADE A DIFFERENCE OF 560 FT. THE THOUGHT CROSSED MY MIND THAT A VFR AT 17500 FT MIGHT BE AT MY ALT AND I ASKED ZAU IF THIS WAS A SAFE ALT WITH THIS HIGH ALTIMETER SETTING? THE RESPONSE WAS 'ORD ALTIMETER IS 30.48.' LATER; DURING THE FLT; I ONCE MORE ASKED ABOUT THE SAFETY OF USING FL180 WITH THIS HIGH ALTIMETER AND WAS TOLD THAT THE ORD ALTIMETER WAS 30.48. BEFORE I LEFT ZAU; I WAS ASKED TO CALL ZAU. WHEN I LANDED AT ORD; I TALKED TO SEVERAL PLTS AND 2 ATC REPRESENTATIVES AND WE ALL QUESTIONED THIS ALT. I LOOKED AT FAR 91.121 AND SAW THAT FL180 WAS USABLE WITH AN ALTIMETER OF 29.92 OR HIGHER. I CALLED A MR X AT ZAU AND WE DISCUSSED THE PROBLEM. HE SAID THAT THEY WERE TRYING TO SEE IF THIS WAS AN UNSAFE PROC; BUT HIS CTLR FELT THAT I HAD BEEN UNREASONABLE ON THE RADIO DURING A BUSY TIME ON THE FREQ. MR X. SAID THAT HE WOULD LISTEN TO THE TAPE AND I SAID THAT I HAD MADE A REASONABLE EFFORT TO ONLY QUESTION DURING A BREAK AND NOT DISTURB THE CTLR. LATER; I TALKED TO ANOTHER PLT; AND WE DECIDED THAT MY REASONING WAS WRONG AND I HAD MORE SEPARATION RATHER THAN LESS AS I HAD FIRST THOUGHT. I CALLED ZAU AGAIN AND TRIED TO CONTACT MR X BUT WAS NOT SUCCESSFUL. THE PERSON I TALKED TO SAID HE WOULD RELAY TO MR X THAT I HAD CALLED AGAIN AND HAD FIGURED OUT THERE WAS NO SAFETY ISSUE INVOLVED. I DON'T KNOW IF THE CTR IS GOING TO ASK FOR ANY ACTION TO BE TAKEN. COMMENTS: SINCE SO MANY PEOPLE WERE CONFUSED ABOUT THE SAFETY INVOLVED AND I DO NOT CARRY FARS OR THE AIM ON BOARD THE ACFT FOR GUIDANCE; I FEEL THAT THE RESPONSE FROM THE CTR WAS NOT CALLED FOR. ALL THE CTR HAD TO DO WAS SAY THAT THEY WOULD CHK AND EVERYONE WOULD HAVE BEEN SATISFIED. I THINK THAT THERE MIGHT HAVE BEEN CTLR TRAINING IN PROGRESS AS SHE SEEMED TO BE HAVING A HARD TIME AND WAS OVERCTLING TO A DEG. RECOMMENDATIONS: I AM GOING TO SEE IF WE CAN INCLUDE THE CHART FROM THE FARS IN OUR FLT OPS MANUAL SO THAT A PLT CAN QUICKLY FIND GUIDANCE WHEN THERE IS A QUESTION ABOUT A SAFE ALT.