ACR WDB GLASS COCKPIT ACFT FO EXHIBITS DOUBTS AS TO ACCURACY OF ACFT WTS VS SPDS ON COMPLEX RWY CONDITION WET SLUSH TKOF COMPUTATIONS.

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PREPARING FOR DEP FLT MAR/THU/92; JFK-NRT. SLUSH 1/4 INCH RPTED ON DEP RWY 4L. WE CHKED THE WT/PERFORMANCE CONCERNING SLUSH/RWY CONTAMINANTS FOR WT RESTRICTIONS AND REDUCTION OF V1 SPDS. GOT THE OFF DUTY FO TO RUN THROUGH THE MANUALS/CHARTS TO CHK FIGURES BASED ON OUR CLOSEOUT MLG MESSAGE. WE WERE BELOW THE ALLOWABLE TKOF GROSS WT AND FIGURED V1 VERSUS VMCG SPDS OFF ANOTHER CHART CAUSED SOME CONFUSION AS TO WHICH APPLIED AND WHERE WE STOOD ON THE PRESENT RWY FOR MAX WT. LAST CHK WITH TWR STATED BRAKING ACTION GOOD BUT RWY WAS NOT CLRED COMPLETELY. OFF DUTY FO AND ON DUTY CAPT AFTER RETRACING THE STEPS THROUGH THE CHARTS; AGREED THAT WE WERE 'GOOD' TO GO. AFTER DEP (UNEVENTFUL) AND CLBOUT; REVIEWED THE PROBLEM AGAIN AND I WAS STILL UNSURE IF WE WERE BELOW THE PROPER GROSS WT. I WAS NOT AGGRESSIVE ENOUGH DURING THE PROBLEM AND WE SHOULD HAVE GOTTEN DISPATCH INVOLVED IN THE SITUATION PRIOR TO TKOF. THE OFF DUTY FO IS AN INSTRUCTOR AND TYPE RATED ON THE EQUIP; THEREFORE; DEEMED HIGHLY QUALIFIED AND SEEMED SURE THAT ALL WAS OK PRIOR TO DEP. IT WAS ONE OF THOSE 'RARELY COMES UP' CASES OF ACFT PERFORMANCE PROBLEMS. IT WOULD ALSO HAVE BEEN MUCH EASIER TO BE 'IN THE LOOP' PRIOR TO ACFT MOVEMENT. WE DIDN'T RECEIVE OUR LOAD NOTICE UNTIL AFTER TAXI PLUS THE PUSH TO BE ON TIME; AND RELIANCE ON ADDITIONAL CREW MEMBERS ASSIGNED TO LONG HAUL FLT COMBINED TO RESULT IN MY 'RELIANCE ON THE OTHER GUY.'