ACR B727 FLC INITIALLY DETERMINE TWR CLRNC FOR TKOF ALLOWED THEM TO TAXI ONTO INTERSECTING RWY FOR DEP DUE TO TXWY CLOSURE. THOUGH FLC DISPUTE CLRNC; AND HOPED FOR A MORE DEFINITIVE CLRNC; THEY CONCLUDE THEY WERE TECHNICALLY IN ERROR.

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INCIDENT OCCURRED DURING TAXI FROM GATE TO RWY 11. TXWYS A AND F WERE CLOSED. AVAILABLE ROUTING WAS RAMP; TXWY E; RWY 7/25; BACK TAXI ON RWY 11/29; DEP END OF RWY 11. GND CLRED US TO TAXI TO RWY 11. WE PROCEEDED ALONG THE ABOVE MENTIONED ROUTING; INCLUDING BACK TAXI ON RWY 11/25. THIS WAS THE ONLY AVAILABLE TXWY TO RWY 11. APPROX 2500-3000 FT DOWN RWY 11/25; WHILE BACK TAXIING; TWR/GND QUERIED US ABOUT CLRNC TO ENTER RWY 11/25. WE EXPLAINED OUR UNDERSTANDING OF THE CLRNC AND APOLOGIZED. TWR/GND THEN ISSUED US BACK TAXI CLRNC. THERE WAS NO CONFLICT WITH ANY OTHER GND OR AIR TFC. IN FACT; THERE WERE NO OTHER ACFT ON EITHER FREQ. IN HINDSIGHT; I BELIEVE THE CTLR WAS CORRECT IN HIS CLRNC AND WE SHOULD HAVE HELD SHORT OF RWY 11/25 FOR FURTHER CLRNC. HOWEVER; IN THE INTEREST OF CLARITY AND SAFETY THE CTLR COULD HAVE INCLUDED '...HOLD SHORT OF RWY 11/25 FOR BACK TAXI CLRNC;' TO HIS TAXI INSTRUCTIONS. THIS STATEMENT WOULD HAVE REMOVED ALL DOUBT IN WHAT HE WANTED. AIR-TIME WAS NOT A FACTOR. OUR FLT WAS THE ONLY ACFT ON FREQ AT XA55 IN MISSOULA; MT.