A B737 PIC STARTS A PREMATURE DSCNT PRIOR TO CORAM INTXN ON THE RWY 24 ILS APCH PASSING CCC; NY.

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AFTER PASSING OVER CALVERTON VOR (CCC) ON THE ILS RWY 24 AT ISP I LOOKED DOWN AT THE PROFILE VIEW AND MISTOOK THE CORAM INTXN DEPICTED FOR CALVERTON VOR AND INITIATED A DSCNT FROM 2000 FT TO 1500 FT (I WAS FLYING). THE FO DID NOT SEE ME INITIATE THE DSCNT. AFTER LEVELING OFF HE NOTICED IT AND TOLD ME WE WERE NOT PAST CORAM AND WE SHOULD STILL BE AT 2000 FT. I IMMEDIATELY INITIATED A CLB TO 2000 FT. AT THE SAME TIME APCH CTL TOLD US WE SHOULD STILL BE AT 2000 FT. WE HAD BEEN CLRED FOR THE APCH APCHING CALVERTON VOR. WE HAVE A COMPLACENT WAY OF DOING APCHS HERE AT COMPANY. NORMALLY THE PNF SETS THE NEXT ALT IN THE WINDOW; BUT WHEN WE START THE APCH THE PF STARTS DOING IT EVEN WHEN THE PNF ISN'T LOOKING. THAT CONTRIBUTED. ALSO THE APCH PLATE IS CONFUSING. WITH CCC IN THE PLAN VIEW BUT NOT IN THE PROFILE VIEW; I DON'T KNOW IF THAT CAN BE FIXED OR NOT. SUPPLEMENTAL INFO FROM ACN 536216: PASSING CALVERTON (CCC) NY APCH CLRED US FOR THE APCH; ILS RWY 24. I LOOKED DOWN TO CHK THE RADIAL; DIALED THE COURSE INTO THE COURSE SELECTION WINDOW ON THE MSP (271 DEGS) AND THEN SET UP THE NEXT FREQ ON VHF #1 (LONG ISLAND TWR). WHEN I RETURNED MY ATTN TO THE PERFORMANCE INSTS I NOTICED WE WERE DSNDING PAST 1700 FT. I IMMEDIATELY CHKED THE ALT WINDOW ON THE MSP AND SAW 1500 FT. I DIDN'T THINK WE WERE SUPPOSED TO DSND TO 1500 FT UNTIL CORAM ABOUT 4 MI AWAY. THE FOM STATES THE PNF SHOULD MAKE ALL CHANGES TO THE ALT SELECTED ON THE MSP. HOWEVER; MANY COMPANY PLTS DIAL UP THEIR OWN ALTS WHEN ON THE APCH; ARRS INTO LAS; LAX ESPECIALLY COME TO MIND.