Gidday Howard. That number is based on ISA temperature I would assume. For about 90% of the year, Jandakot would be above ISA. Significantly above for about 50% of the year, I would say.
Assuming the 1.67 factor, I would say there would be too many occasions when a landing at YPJT could not legally be made.
Correct ISA, but aerial work does not require factoring.
From CAO 20.7.1B
11.3 For subparagraph 4.1 (d) and paragraph 5.1, an aeroplane engaged in private, or aerial work, operations must be operated so that compliance with the landing requirements is demonstrated using data set out in:
(a) the flight manual; or
(b) the manufacturer’s data manual; or
(c) the approved foreign flight manual.
Note: The data contained in some manufacturers’ data manuals is unfactored and makes no allowance for degraded aircraft performance.
Source: casa.gov.au