Thousands of flight are operated every day using this certified app, so I think it's reliable.
Can you please explain again your calculations, what do you mean by double derate ?
Initial conditions for the experiment:
R-24K 31°C 93.4% Airport is RAK, RWY10, DRY, 0 WIND, 15°C, Q1015, Flaps 5, A/C AUTO, A/I OFF
Engine inop go distance 2165M
ASDR 2238M
all engine go distance 2210M
1- Change
R-26K 45°C 93.4% Airport is RAK, RWY10, DRY, 0 WIND, 14°C, Q1015, Flaps 5, A/C AUTO, A/I OFF
Engine inop go distance 2288M
ASDR 2366M
all engine go distance 2343M
2-Change Airport is RAK, RWY10, DRY, 0 WIND, 16°C, Q1030, Flaps 5, A/C AUTO, A/I OFF
R-26K 46°C 93.4%
Engine inop go distance 2254M
ASDR 2331M
all engine go distance 2306M