Ex Douglas Driver,
Unless your aircraft has a very benign first segment performance, your post frightens the living daylights out of me ... sadly it reflects the generally abysmal level of performance knowledge amongst many of the piloting fraternity. It is unfortunate that the Australian fraternity doesn't have to wade through the likes of Perf A as the traditional CASA (and precedent organisations) performance exams (and the related Industry training courses) tend to gloss over the nitty gritty of getting off and away from the bumpy bits. One needs to keep in mind that the first segment may well be the critical takeoff consideration .. albeit that a lot of folk ignore it to a greater or lesser extent.
(a) having done a LOT of work in the past on precisely the same problem with some quite limiting first segment aircraft, I suggest that the 60m/20m possibly are figures plucked out of thin air .. are you able to provide a bit more meat as to the rationale and engineering logic ? Certainly those sort of numbers make no sense when viewed against the geometric constructions I've done in the past ...
(b) a bit risky to mix TORA and TODA/STODA .. bit like apples and oranges.
(c) if you elect to make an artificial "TORA" exceeding the runway declared TORA, who authorises such a practice ? Not sure that I would like to be on the receiving end of that one from a skilled barrister in court after the accident ...
The simplest way to achieve your goal is one of the following ..
(a) make the weight such that the first segment fits in the available TOD. Generally, this is only suitable for one-off get-me-home-quick exercises as it is usually very conservative.
(b) make the weight such that the available first segment gradient is not less than the required obstacle clear surface gradient. Likewise very conservative.
(c) do the obstacle analysis .. quite easy, really .. it certainly is not correct to say that the obstacle data is unknown .. but you might have to do a bit of legwork to track it down ... options -
(i) get the obstacle data .. approach the airport owners and/or the surveyor who did the survey .. often free if one asks nicely but might cost a few dollars or a couple of beers .... or
(ii) work a reverse engineering analysis to figure the STODA/TODA intersections from the RDS data. This will not be entirely accurate with regard to the obstacles due to round off in the ERSA data but will be a bit conservative so all's well. In general, the calculated data is reasonably close to the actual critical obstacles which formed the basis for the RDS figures .. I don't think anyone has done a simple inclino survey for many years, if ever ? Certainly, playing with AFMs over the past thirty odd years as a consulting engineer, I haven't come across one .. always been a discrete survey.
(iii) with the obstacle set (either real or inferred) it is a simple geometric exercise to fit the AFM data to the obstacle profiles
Presuming that you fly for a local operator, you might consider suggesting that an appropriate person in the organisation seek professional guidance on the matter .. in the first instance, I guess that Old Smokey or I would be happy to have a quick look at the engineering philosophy as we are both very familiar with the Oz operating and regulatory environment.
Two other thoughts ..
(a) don't forget that the runway slope might just catch up with and smite the aircraft for WAT conditions ..
(b) what does your company do about the third segment acceleration in this general T/O analysis case ?