I filled my oldest TDI (April 2002 with 34L tank) up to the brim last week, using the vent several times to make sure I completely filled it.
448 miles (low-fuel beep plus 20 miles) on 33 litres = 61.7MPG true, where the DIS was showing me 62.3MPG.
Most of that mileage is my 20-mile commute, a mix of 30-limit, rural B-road and 60mph dual carriageway, plus a few short local runs.
Having reset the DIS, 160 miles into the new tankful my commute has got me to an indicated 66MPG without trying too hard.
A steady 60mph for 100 miles on a fresh tank & reset DIS will get me over 70MPG.
My Cobalt TDI (November 2002 with 42L tank) at the same mileage into a full tank is just about holding an indicated 59.3MPG on my commute, following 120 miles of mostly 60mph dual carriageway. This disappoints me slightly in light of the above.
Your figure sounds like it's in the ballpark. However, reversing your calculations, you put 28 litres into a 34L tank. If you filled up soon after you got the low-fuel warning, I would have expected you could have got more in, by using the vent at each shut-off of the pump until the level didn't go down any more. Using this method for two consecutive fill-ups - ideally at the same side of the same pump - and noting the mileage between them will get you the most accurate figure.