Vorshlag has long promoted that you can never have too much camber in your McPherson strut suspension. For the MQB chassis Volkswagens and Audis, this is especially true.
That's a tight tower opening on this Audi TT
Adding camber plates to the Volkswagen is a real commitment. The tower opening is very limiting, more so if the damper adjustment is at the top. So if you are going to this much effort, you are going to want to run a lot of camber and/or caster.
The problem is that the strut tower opening is very small on this chassis (see above). So even with our camber-caster plates, you might not be able to get to the ideal camber or caster setting with some ride heights or strut brands, as the strut shaft will can into the opening at the strut tower.
This picture above is from a S550 Mustang, but it has a similar shaped tower. Cutting the tower opening larger helps you get to the alignment adjustments your tires might need. And if you want to run any additional positive caster, this way you won't lose camber adjustment.
Some people have really hacked up towers - with a Dremel or saws-all - or made some hole saw drilling fixture out of a block of wood, which is easy to get started wrong and end up with a hole that is off-center.
This is where the Vorshlag Strut Tower Cutting Fixture comes in. This tool bolts into the Focus strut tower from underneath, to help align a hole saw cut. These are CNC cut and welded together on a production fixture, at our facility. Bolt it in, line up your hole saw bit, insert a 2-3/4" hole saw (or bigger if you like) with a 1/4" arbor bit into the fixture, and cut down towards the plate. The raised centering bushing lines up the cut so it is never off-axis. You should be able to use this tool dozens of times, on both sides of the chassis. The saw might scuff the jig from touching the plate, but it's 1/4" steel and can take a beating.
The fixture comes raw and unpainted - we are trying to keep costs low. You supply the top nuts, the hole saw, and use a 1/4" bit for the arbor. Don't worry, we've done this cut on dozens of cars for many years, and have not seen any weakness in the multi-layer towers on these cars.
This link shows how to use use this fixture in the S550 chassis. This same technique works on a number of cars (a McPherson strut design is pretty generic)