A B757 PILOT ABORTED A TKOF AT 80 KTS FOR A FORWARD CARGO LIGHT. SINCE 80 KTS IS THE LOW SPD CUT OFF; IF SPD WERE 81 WHEN THE LT CAME ON THE TKOF WOULD CONTINUE.

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AROUND 80 KTS ON TKOF ROLL RWY 11L AT ZZZ THE AFT CARGO DOOR EICAS MESSAGE ILLUMINATED. THE TKOF WAS DISCONTINUED. WE THOROUGHLY FOLLOWED QRH AND FOM GUIDANCE AND COORDINATED WITH DISPATCH; MAINT; DUTY PLT AND THE CHIEF PLT'S OFFICE. LOOKING AT THE AFT CARGO DOOR; UPON RETURN TO THE RAMP; THE 2 PRESSURE DOORS WERE CRACKED OPEN APPROX 1/2 TO 3/4 OF AN INCH. PRIOR TO THE SECOND PUSHBACK; CONTRACT MAINT; BOTH PLTS AND RAMP WORKERS OBSERVED THE DOOR BEING CLOSED. BOTH PRESSURE DOORS APPEARED SLIGHTLY CRACKED OPEN THOUGH LESS THAN OBSERVED ON INITIAL INSPECTION AFTER RETURN TO THE RAMP. THE DOOR WAS CYCLED ONCE MORE WITH THE EFFORT EXERTED ON THE HANDLE GREATER. THIS TIME THE PRESSURE DOORS APPEARED FLUSH. THE DOOR ANNUNCIATOR SYS WAS PUT ON LOGBOOK PER COMPANY MAINT; THOUGH THE DOOR LIGHT DID NOT RE-ILLUMINATE. THE SUBSEQUENT FLT WAS UNEVENTFUL. WHEN CONDUCTING THE TKOF; WE HAVE 2 REGIMES LOW SPD AND HIGH SPD. BOEING'S QRH SAYS PRIOR TO 80 ABORT FOR X; Y; Z. AT 80 TO V1 ABORT FOR X; Y. ABOVE V1 ABORT FOR X. DURING THE TKOF ROLL THE PNF CALLS OUT 80 KTS. AS THE PF I'M TRIGGERED TO LOW SPD ABORT CONSIDERATIONS UNTIL I HEAR THE CALLOUT. THEN I'M TRIGGERED TO HIGH SPD ABORT CONSIDERATIONS. IN THE ABOVE SITUATION THE CALLOUT AND THE ANNUNCIATOR OCCUR AT THE SAME TIME. MY MIND HASN'T FLIPPED FROM LOW SPD TO HIGH SPD YET BUT FACTOR IN TIME TO ASSESS THE LIGHT; FORMULATE A RESPONSE AND ACT ON THE RESPONSE MEANS THAT THE THRUST LEVER MOVEMENT OCCURRED AFTER 80 KTS THOUGH NOT BY MUCH. THE AUTOBRAKE RTO SYS ACTIVATED AND WE STOPPED WELL; WELL SHORT OF THE END OF THE RWY. SUBSEQUENT INVESTIGATIONS SEEM ONLY INTERESTED IN WAS IT 79 KTS OR WAS IT 81 KTS. BOEING INHIBITS THE MASTER CAUTION SYS AT 80. WAS THE LIGHT ON OR OFF; BLACK OF WHITE? IF THE EVENT HAD OCCURRED 10 KTS EITHER SIDE IT WOULDN'T HAVE BEEN AN ISSUE. ABORT CONSIDERATIONS CANNOT SOLELY BE MADE USING THE MASTER CAUTION SYS. IS THE ACFT SAFE TO FLY AND ENG FAILURE ARE NOT DETERMINABLE SOLELY BASED ON THAT LIGHT. IF THE MASTER CAUTION LIGHT COMES ON THEN GOES OUT DO YOU START TO ABORT THEN SAY NO IT IS OUT NOW I HAVE TO CONTINUE WITH THE ASSOCIATED CLUSTER AROUND THE THRUST LEVER QUADRANT THAT WOULD ENTAIL? THE REQUIRED PLT CALLOUT OF 80 KTS DOES NOT OCCUR EXACTLY AT 80 KTS; EACH AND EVERY TIME. HAVING A 1 KT DETERMINANT OF GUILT OR GENIUS IN THIS SPD RANGE IS NOT CONSISTENT WITH REAL WORLD; DAY TO DAY LINE OPS; OR SAFETY.