This is an immotive subject for me aswell. In the past with different cars etc... I have literally had it tracked at one place, gone up the road to another who say it's out !
The problem seems to be the equipment and set up the garages use, abuse and all that, their machine tells them it's OK but it's not, it's just the best they can do.
I now go to a rally car prep company in Norwich (an 80 mile trip ) as they have one of the only "Sun" set ups laid in tons of concrete and boy is it accurate.
Since then never had issues and whatever car I go in with it comes out fine. My suggestion would be to find a race prep guy near to you as these boys don't mess about as it's serious stuff to them and most are more than willing to take your car in.
They even weight the cars down to the specs to ensure any pre load on the suspension is accounted for before adjusting.
Normal costs vary from around £47 upto £300 if you have KDS done including setting the steering box straight and working out from there although that 's only necessary if you have a rebuilt front end to set up.
The same goes for wheel balancing, make sure you at least have it done on a machine with the studs adapters on not just the cone type.
It may seem a lot of bother but if your like me spending anything from £600-850 for a set of decent tyres the last thing you want is uneven wear.
Cheers
Rich