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density altitude?

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i was showing a student how to calculate for DA and they brought to my attention that DA= PA + (100*(current temp.-ISA)). i was taught, and have been using 120 in my formula not 100. i looked further into it and have found numerous websites using 100 instead of 120. have i been wrong all this time? i was wondering if anyone could clear this up for me?

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I used DA= PA + 120 ( Temp C - ISA) since I was taught that in 1975 or so. I don't know were the 100 came from other than well its 100 and its a quick in you head thing. Close enough I guess. When we did it on E-6B's we would always get small differences in answers. I know the FAA writtens answers to these questions is based on the ASA electronic E-6B. If you round off round off on the High side and give yourself a little fudge factor on your performance charts. All it is is maybe throwing 75 to 100 lbs off the helicopter for a given set of conditions.

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