BACK TAXI ON RWY DUE TO INSUFFICIENT RWY LENGTH FROM INTXN.

Narrative

WHILE TAXIING ON TXWY N FOR TKOF ON RWY 22R; BOSTON TWR CALLED US AND ASKED IF WE COULD DEPART FROM TXWY N1. OUR OPS MANUAL ALLOWS US TO MOVE OUR ACFT 600 FT DOWN THE RWY PRIOR TO INITIATING THE TKOF ROLL AND A TKOF PROC IS SPECIFIED. WE INITIALLY ASSUMED THAT DEP FROM N1 WOULD BE WELL WITHIN THIS 600 FT; AND TOLD TWR WE COULD DO IT. UPON CLOSER EXAMINATION OF THE ARPT DIAGRAM; WE REALIZED THAT TXWY N1 WAS MORE LIKE 900 FT FROM THE DEPARTING END OF RWY 22R. BY NOW; WE WERE ON TXWY N1 AND SHORTLY THEREAFTER CLRED INTO POS. IN ORDER TO MEET OUR 600 FT CRITERIA; WE HAD TO BACK TAXI TO WITHIN 600 FT OF THE END. AFTER DOING SO; WE DEPARTED USING THE APPROPRIATE TKOF PROC. THE DEP WAS NORMAL. THESE LAST SECOND REQUESTS FROM TWR CAN TRIP UP A CREW THAT HAS ALREADY COMPLETED ITS TKOF AND PERFORMANCE CALCULATIONS BASED ON PREVIOUS INSTRUCTIONS. IT WOULD HAVE BEEN HELPFUL TO US IF WHEN TWR MADE ITS REQUESTS TO USE N1 INTXN THE CTLR COULD HAVE TOLD US HOW MUCH RWY LENGTH WE WOULD BE LOSING. THIS WOULD HAVE SAVED A LAST MIN SCRAMBLE FOR INFO BY THE FLC; THE ESTIMATION OF THE 600 FT POINT AND THE RESULTING BACK TAXI.