my preference for density altitude computation is to determine the field's deviation from standard temp and multiply that by 120'. Add or subtract that to/from pressure altitude and you have an accurate density altitude. In other words, each 1 degree C affects density altitude by 120'. Ex. Field elevation 5,000', temp. 30C, altimeter 30.17.
Pressure alt.= 4750'
standard temp at 5,000' = 5C
30 (current temp) - 5 (standard for the field elev)= 25 (degrees above standard)
25 * 120 = 3000' (this is your correction for non-standard temp)
3000' + 4750' = 7750' (density altitude)
From this you can see that simply going to the next 1000' higher on the pressure altitude for the landing distance chart will cover you for just shy of 9 deg. above standard temp.
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