A CARJ CAPT DEVIATED; WITHOUT PERMISSION; AROUND TWRING CUMULUS IN A80'S AIRSPACE CAUSING A POSSIBLE SEPARATION ISSUE.

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INBOUND TO ATL ON THE LA GRANGE ARR; ATC ISSUED 090 DEG VECTOR. WX WAS VMC WITH TWRING CUMULUS BUILDUPS. I BEGAN TURN WITH AUTOPLT; USING HDG BUG FROM APPROX 030 DEGS TO 090 DEGS. AS ACFT WAS COMING AROUND I SAW THAT HDG WOULD TAKE US THROUGH A CUMULUS BUILD-UP. I SLEWED THE HDG BUG FURTHER TO ABOUT 120 DEGS TO CONTINUE THE TURN; AND THEN WHEN I KNEW WE WOULD CLEAR THE BUILD-UP; I RETURNED THE HDG BUG TO 090 DEGS. THE ACFT HDG REACHED APPROX 110 DEGS BEFORE REVERSING DIRECTION. ATC THEN CALLED AND ASKED OUR HDG. THE FO REPLIED 'RETURNING TO 090 DEGS AFTER A SHORT DEV.' ATC WAS NOT HAPPY AND SAID WE NEED TO TELL HIM BEFORE DEVIATING BECAUSE DEPARTING TFC IS RIGHT THERE CLBING THROUGH OUR ALT. THE DEV OF 20 DEGS OR SO FOR ONLY A MI OR 2 DID NOT SEEM LIKE ENOUGH TO BE NOTICED. WITH ATC FREQ CONGESTION IT IS ALMOST IMPOSSIBLE TO REQUEST SMALL DEVS IN A TIMELY ENOUGH MANNER. ALSO IF ARR AND DEP PROC CHARTS CONTAINED SOME INFO ON AIRSPACE ARCHITECTURE IT MAY HELP PREVENT TRANSGRESSIONS INTO BORDERING AIRSPACE.