Ezy,
I dont know where your original source of info came from but the N2 does not make sense to me and should actually be Ng.
The incident that I referred was a 350B1 with a 1D engine which was doing repetitive lift jobs with periods on the ground at idle. Partial cycles were not being recorded and the gas generator assembly eventually "left the building" so to speak.
General consensus is that recording of partial cycles has not been recorded so well in the past as it is quite difficult to achieve accurately without some assistance from automation. Many operators got a bit of a wakeup call when migrating from B2 to B3 which of course does it automatically! This is evident mostly with repetitive heavy lifting.
If you dig really deep in 350B SB's and TM TU's there is/was a tacho box modification that will/would do it for you. That being said there are aftermarket units out there that may be more economic as that mod may have disappeared.
The trick with the formula is knowing the maximum Ng achieved for a particular flight plus the number of times the Ng goes below 70%. If you have a flight profile that is pretty standard it is not so difficult but other profiles can be a distraction.