There's no hard and fast rules about what speed to do in what gear etc. For a start, different models have different gearing, and we all have a different driving style and preferences for how quickly we accelerate.
If you're just driving normally, just change up when the engine can comfortably take the next gear without struggling.
Driving at very low rpm is counter-productive even if it's not rumbling, TDIs are less efficient when running off-boost. Peak efficiency is where peak torque is, when the turbo is happily doing it's job and the ECU isn't having to overfuel regularly to get the turbo back up to speed when it drops off boost.
My
Ibiza used to be far more efficient when cruising gently in, say 5th at 1900-2000rpm than it would be if I changed up to 6th and the revs dropped to 1500 or below.
With regards to changing into second - during day to day driving I use first just to get moving then change into second, going any higher in first just makes for a jerkier drive. Unless you're racing someone off the lights...
For absolute max acceleration it's a different matter, it all depends on your engine's power curve. Then you're simply aiming to use the rev range between gear changes that would have the largest area under it on a graph of RPM vs Power. Generally this would mean changing up a little way after peak power has been reached, so that in the next gear your start below peak power, go through it and past again; for most it will mean reaching somewhere between 4000-4500 RPM before each change.
On remapped cars you might be able to go higher, depending on the map and mods, as they generally maintain torque (and therefore power) longer up the rev range, where standard maps often tail off very significantly past 4k rpm.