nigel just remembered
the man to my right has just kindly reminded me that if your computer does not have an input for relative humidity or dew point it defaults to believing that at 43 deg C the rel humidity is 34% and at 15 deg C it believes rel humidity is 80% with a linear variation between these values which are treated as end-stops. i do remember it come to think of that and if you play with richard shelquist's lovely on-line density altitude calculators (he calls himself "wahiduddin" you'll see what merry heck gross errors make on your take-off performance. doncha just hate it when the man in the right seat is smarter than you?