JT, you've confused some assumptions I provided, for the purposes of simplicity, for assertions of fact, then added your own assumptions that were not made in the questions.
For the record, I know that:
- 1013 hPa isn't the precise sea level pressure in the ISA
- no altimeter is perfect
- an aircraft's static source isn't necessarily at exactly the same level as the altimeter/s to which it is input, and
- etc
all of which looks to me very much like 'splitting of hairs' by you, unless that all adds up to 100's of feet of error? (I did spend the odd decade or so working on avionics, BTW.)
You've added your own assumptions that weren't in the questions I asked. You've assumed that we "are nearby the departure aerodrome" and "remain in the same forecast QNH area". So when you "note" that a figure is "incorrect", it's based on your assumptions. I could assume that BOM's forecast Area QNHs are accurate to within plus or minus 0.1 hPa, but that would be imprudent because, as a matter of fact, BOM states that "[a]n altimeter set to Area QNH will be representative to within ±5 hPa of any actual QNH of any location within the defined area."
You are flying YCBP - YFRT - YPKG. You have forecast Area QNHs for the 4 Areas through which you're going to fly. Let's pick 2,500' 'indicated' on the altimeter as the cruising altitude. What do you say is the range of heights above MSL within which the aircraft will definitely remain during the cruise? Make whatever assumptions you want about the height of static source compared with the altimeter and lapse rates etc. If you can bring yourself to do it, assume you nail the long pointer on the altimeter to the '5' for the entirety of the cruise.
2,500' plus and minus what? PI insurance level of confidence.