Good job!
First / next, fit the gauge at the MANIFOLD, and not just a Tee-Piece at the turbo end. This will show you the real boost at the engine.
This is exactly what I've always done with any turbo boost gauges.
At the rear of the TD5 inlet manifold is a bog standard steel core plug. This is the ideal place to fit your take off tapping as it is completely protected and fully reversible just by fitting a new core plug. No drilling of manifold required.
Knock the core plug
INTO the manifold. If you try to knock it out it will bite into the soft alloy manifold and bugger it. You need to stop the core plug dropping into one of the manifold inlet tracts so this is a lot easier with the manifold off the engine. However I did mine by sliding a long cardboard tube into the manifold via the inlet opening to catch the core plug and then withdrew the cardboard tube with the core plug safely inside.
Another method is to glue a strong magnet to a wooden dowel and withdraw the core plug that way.
I drilled the core plug to fit the boost gauge brass tapping (which should come with the gauge) and brazed it in place.
Clean up the manifold orifice and then lightly smear the core plug with some Hermetite blue before gently tapping it back into place. Wipe off any excess sealant and connect up the gauge - Job done.
If you want to go even further you could feed the turbo wastgate from here rather than from the compressor housing but I never got round to this. This would make the turbo slightly more efficient as it will be using actual manifold pressure to regulate the boost. Its what they do on race cars as standard.
I'm not sure about the 200/300 tdi manifolds but the principle is the same.
