LTT AIR AMBULANCE CAPT MADE A COMPUTATION ERROR IN CALCULATING LNDG PERFORMANCE DISTANCE REQUIRED.

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DEPARTED ZZZ FOR XXX WITH 4000 FT OF RWY AVAILABLE AT ZZZ. TKOF FIELD LENGTH CALCULATIONS HAD BEEN DONE BY PREVIOUS DUTY CREW AND I DISCUSSED THEM WITH THE PREVIOUS DUTY CAPT AND CONFIRMED THEY WERE CORRECT. I DISCUSSED WITH THE OTHER CAPT THE LIKELIHOOD WE WOULD HAVE TO RETURN WITH PATIENT TO XXX BECAUSE OF LIMITED RWY AT ZZZ. WHEN FLT WAS CALLED AND DISPATCHED; I RECHKED NUMBERS; CALCULATED FROM PERFORMANCE CHARTS IN CHKLIST THAT WE WOULD REQUIRE LESS THAN 2800 FT TO LAND UPON OUR RETURN TO ZZZ AND USING 2 DIFFERENT CALCULATORS CONCLUDED THAT MET THE PART 135 60% RULE. DURING OUR WAIT FOR PATIENT AT XXX; (AGAIN CALCULATED LNDG DISTANCE OF LESS THAN 2800 FT AND AGAIN MISCALCULATED THAT AS 60% OF THE 4000 FT AVAILABLE). I SHOWED MY CALCULATIONS TO MY COPLT AND CONFIRMED WITH OPS THAT WE WOULD BE FINE WITH ONLY 4000 FT AT ZZZ. ON LNDG AT ZZZ WE TAXIED OFF OF RWY AT TXWY A11 WITH LITTLE BRAKING REQUIRED. THAT LEFT OVER 1100 FT OF RWY REMAINING. THE RWY CLOSURE WAS AT THE SAME LOCATION (ABEAM THE S EDGE OF TXWY A8; WITH RED TAPE ALONG THE RWY EDGES FOR APPROX 200 FT AND A LARGE; ELEVATED LIGHTED 'X' ON THE CTRLINE. 2 FLASHES OF ILLUMINATION STRUCK ME AS I WROTE THE ABOVE. THE CHARTED LENGTH FROM TXWY A8 TO THE END IS 5000 FT; AND WE WERE AT 50 FT RA AS WE CROSSED THE LINE ACROSS THE RWY SIGNIFYING THE CLOSED PORTION. I CAUGHT THE ERROR IN MY CALCULATIONS WHILE REVIEWING MY PAPERWORK AFTER THE FLT. WE DID; INDEED; STOP WITHIN 60% OF THE AVAILABLE RWY WITHOUT EXTRAORDINARY PLTING EFFORT; AND I DO NOT BELIEVE ANYONE WAS SUBJECTED TO ANY ADDITIONAL HAZARD. I HAVE RECALCULATED THE NUMBERS I USED FOR THE DECISION MAKING PROCESS; AND WAS ABLE TO DUPLICATE MY ERRONEOUS OUTCOME BY MULTIPLYING BY .7 VICE THE REQUIRED .6. I BELIEVE I AM NOT LIKELY TO REPEAT THAT MANY KEYSTROKE ERRORS EVER AGAIN.