I have been comparing Sky Demon Plan 3.7.1.0 with the 1/2 mil chart.  There is a section of Papa 600 south of Perth FL 85 and FL55. The alignment of the South Eastern boundary of this section of the airway is different in different in Sky demon to that of the 1/2 mil chart.  Which is correct?
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Have you checked the AIP?

No Tim and at present have no intention of doing so. I use Sky Demon software for navigation and to avoid infringing controlled airspace.  Maybe not the way it is done today but I also carry a 1/2 mil chart so it is important that controlled airspace is shown correctly on both.  The alignment of the airway I described as shown on the 1/2 mil charts has not changed for at least the last 20 years.  Being the new kid on the block the error is most likely in Sky Demon. 
As best I can I have described the location where the differences occur with the official 1/2 mil chart.  It is the section of  P600 south of Perth where the base of the airway is FL85 and FL55.  The SE boundary of the airway as shown on the 1/2mil chart is further to the west than that shown in Sky Demon. 
‌Just thought that Sky Demon may wish to check this out to prove the integrity of their navigation software. 

I have indeed checked it, and as far as I can tell we are showing the airway exactly as the AIP defines it. I assume you're referring to the turning point at 561400N 0033819W where the base level changes from FL55 to FL85. There is no nonstandard base level change path at that point.

You state the airway hasn't changed for 20 years but please bear in mind airways are changed every 28 days, and this one has definitely been touched by those changes in recent months.

Possibly the author of the 500k chart you are comparing with assumed the airway base level change would be aligned with the northern boundary of the Scottish TMA (which would make sense) but there is no suggestion of that in the AIP.‌

Pleased to see that following recent chart update the alignment of P600 matches that on the 1/2 mil chart.