ACR TURBOJET DEVIATES FROM CLRED TRACK DUE TO COCKPIT DISTR IN CPR AIRSPACE.

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WE WERE CLRED TO DSND TO 13000 FT AND INTERCEPT THE 29 DME ARC FOR AN ILS APCH TO RWY 3 AT CASPER; WY. WE WERE IFR THROUGHOUT THE DSCNT AND INITIALLY PICKED UP LIGHT MIXED ICING. LATER WE ENCOUNTERED MODERATE MIXED ICING AND I HAD THE WING ANTI-ICE OPENED AND ADVANCED THE THRUST LEVERS TO 70% AS PER THE ACFT FLT MANUAL. THE FO WAS PF AND HE QUERIED ME ABOUT THE INCREASED PWR SETTING AND SAID IT SHOULD ONLY BE 55% N1. WE WERE AT 49 DME; SO I PULLED OUT THE QUICK REF CARDS AND SHOWED HIM IT WAS 70% N1 AND WE DISCUSSED ANTI-ICE USE. WHEN I LOOKED AGAIN AT THE DME IT WAS 23 AND I TOLD THE FO TO TURN IMMEDIATELY TO REINTERCEPT THE ARC. APCH CTL QUERIED US AT THIS TIME ASKING US IF WE WERE GOING TO INTERCEPT THE ARC. IN INFORMED THEM WE WERE IN THE TURN CORRECTING BACK. WE INTERCEPTED THE ARC AND THE REST OF THE APCH WAS UNEVENTFUL. LESSON LEARNED IS THAT I SHOULD HAVE JUST SAID TO THE FO WE WERE GOING TO USE 70% N1 FOR ANTI-ICE AND WE WOULD DISCUSS IT ONCE ON THE GND. THIS WOULD NOT HAVE CAUSED THE DISTR RESULTING IN PASSING THROUGH THE ARC AND OUR HAVING TO REINTERCEPT IT.