FLC MISSES XING ALT ON DSCNT INTO STL.

Narrative

INBOUND TO ST LOUIS; WERE CLRED BY ZKC TO CROSS 12 MI W OF FTZ AT 15000 FT MSL. THE FO (PNF) MISTAKENLY DIALED IN 12000 FT IN THE ALT ALERTER. AT APPROX 12800 FT WE WERE SWITCHED TO ST LOUIS APCH WHO QUESTIONED OUR ALT CLRNC. AT THIS TIME WE REALIZED WE WERE SUPPOSED TO BE AT 15000 FT AND DISCOVERED THE ERROR IN THE ALT ALERTER SETTING. APCH CTL ASSURED US OF 'NO PROB' AND CLRED US DOWN TO 7000 FT FOR THE REMAINDER OF THE ARR. A CONTRIBUTING FACTOR TO MY MISSING THE MISTAKEN ALT SETTING WAS THE 'A' POS FLT ATTENDANT HAD JUST ENTERED THE FLT DECK TO TAKE REFRESHMENT ORDERS DURING THE CLRNC FROM CTR. AFTER HEARING AND RESPONDING TO THE CLRNC; I DIRECTED MY ATTN TO THE FLT ATTENDANT INSTEAD OF FOLLOWING UP THE PROPER SETTING OF THE ALT ALERTER. THE FLT CONTINUED TO STL UNEVENTFULLY. SUPPLEMENTAL INFO FROM ACN 328074: AS A CONTRIBUTING FACTOR I FEEL THE PRACTICE OF READING BACK CLRNCS IN A BRIEF FORM SUCH AS 'CROSS 12 W AT 15' IN AN EFFORT TO SPD THINGS UP IS A POOR PRACTICE AND I WILL NOT DO IT ANYMORE.