A320 CREW HAD A DEP HDG QUESTION AFTER TKOF ON RWY 1 AT DCA.

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THOROUGH PREFLT BRIEF FOR FLT NOTED NEW 10-7 D/E PAGE FOR DCA; ESPECIALLY DEPART NOTE 2 SAYING 'EXPECT' TO MAKE L TURN TO INTERCEPT/FLY DCA 328 DEG RADIAL OFF RWY 1. NORMAL TAXI AND LINE UP ON RWY 1. TWR ISSUED CLR TO TKOF WITH NO FURTHER INSTRUCTIONS. IMMEDIATELY AFTER ROTATION CAPT (PF) ASKED IF WE SHOULD GO RWY HDG. I CALLED TWR TO ASK; THEY SAID NEGATIVE. WE IMMEDIATELY TURNED L TO INTERCEPT DCA 328 DEG RADIAL. WE DID NOT TRANSGRESS P56; IN FACT; DID NOT EVEN REACH THE CTR OF THE POTOMAC. HOWEVER; TWR SEEMED UPSET THAT WE HAD TO ASK ABOUT HDG AFTER TKOF. DURING THIS DISTR; WE DID OVERSHOOT DCA 328 DEG RADIAL TO THE W AND CORRECTED BACK. 10-7E PAGE NOTE 2 LED US TO BELIEVE THAT TWR WOULD ISSUE A SPECIFIC INSTRUCTION ON WHEN TO INTERCEPT. WE SHOULD HAVE CLRED THIS UP PRIOR TO TKOF ROLL; THEN WE WOULD NOT HAVE BEEN DISTR AIRBORNE AND DEVIATED W OF DCA 328 DEG RADIAL. ALSO; DCA 10-7E PAGE NOTE 2 SHOULD STATE IN NO UNCERTAIN TERMS TO IMMEDIATELY TURN AND INTERCEPT; NOT TO 'EXPECT.' THIS WAS CAPT'S AND MY FIRST DCA FLT SINCE SEP/01.