Sora front derailleur set up I'm trying to get my front derailleur to stop rubbing in the high gears, smallest sprockets. To do this I am attempting to follow this guide by shimano:

http://bike.shimano.com/media/techdocs/content/cycle/SI/SORA/SI_5GR0A_001/SI_5GR0A_001_En_v1_m56577569830614926.pdf

It says that when in the largest chainring it should be possible to adjust the derailleur so that it is 0-0.5mm aways from the largest chainring on the side furthest away from the bike. This however is not possible and seems to have been set by the lbs simply by putting the cable at very high tension to push it over so it doesn't rub. I believe this may be slipping or stretching and causing the rub to return.

What's the correct way to do this? Is this normal or is the derailleur being dysfunctional?
Wow what a horrible guide.

Tightening the gear cable while in the highest front gear is very tough and will only hurt your fingers. I recommend putting the front dérailleur into the lowest gear and tightening it that way. This is much easier!

As for your specific problem, in all my experience you can never not get 'rubbing' (if by rubbing you mean the chain running through the dérailleur over hang and brushing it). 
However, you will notice that the brushing varies depending on whether the back gears are high or low. Since this varies the angle at which the chain 'points' to the back gears.

So the solution:
Since you will always have brushing (a tiny bit at least), you just seek to minimise it. Put the back gears in the most used gear before tuning the front dérailleur.
For example on my touring bike I'm usually in my third highest gear at the back, unless going up hills or firing down them. So I tighten my front dérailleur cable while the back gear is as such, this means I only get front gear rubbing when the back gear is perhaps in the very low gears, thus causing the chain to 'go backwards' at a different angle.

If you want to spend a ridiculous ton of money on a fancy dérailleur you can mitigate this, but there's really no need.
For mountain bikes the most common gear back gear is the lower one. If you're not sure which  you've used most, clean the cassette of a bike you've used well, and look at the wear on each  cog.