ACR ARR IS ISSUED A DSCNT TO 4000 FT AND MOMENTARILY DROPS BELOW THE TCA BASE.

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WE WERE PROCEEDING INBOUND ON THE BIG SUR ARR. APCH REQUESTED THAT WE LET THEM KNOW WHEN WE HAD THE ARPT INSIGHT. WHEN WE CALLED SFO IN SIGHT; WE WERE CLRED THE TIPP TOE 28L. SINCE OUR PREVIOUS CLRNC HAD BEEN TO 6000 FT; AND THE TIPP TOE INDICATES A MIN ALT AT MENLO OF 4000 FT; WE RESET THE ALT IN THE AUTOPLT TO 4000 FT AND ADVISED APCH OF OUR INTENTIONS TO CONTINUE DSCNT TO 4000 FT. APCH ACKNOWLEDGED OUR CALL. A MOMENT LATER; I REFED THE TCA CHART AND NOTED THAT THE FLOOR OF THE TCA WAS 6000 FT UNTIL 20 NM OF SFO. WHEN I REFERRED TO THE SFO DME; WE WERE 5300 MSL AT 16 NM. WITH A NORMAL PROFILE; WE SHOULD HAVE REMAINED ABOVE THE FLOOR OF THE TCA AS WE DSNDED THROUGH 6000 FT; BUT SINCE I WAS LATE CHKING - I'M NOT CERTAIN. ALTHOUGH THE TCA CHART IS CLR ENOUGH; TRANSITIONING FROM THE BIG SUR ARR TO THE TIPP TOE VISUAL 28L PROC; THE POINT AT WHICH YOU CAN LEGALLY DSND FROM 6000 FT TO 4000 FT IS AMBIGUOUS WITHOUT HAVING 3 DIFFERENT CHARTS OUT. THIS IS AT A TIME WHERE OUR ATTN SHOULD BE PRIMARILY OUTSIDE THE COCKPIT INSTEAD OF CROSS REFERENCING CHARTS. AN ADDITIONAL NOTATION ON THE TIPP TOE 28L CHART SHOWING THE TCA FLOOR CHANGING FROM 6000 FT TO 4000 FT ALONG THE OAK 151 DEG RADIAL WOULD BE BENEFICIAL.