@Bainer I’d say that the short answer to your question is that you should play the lesson slow enough that you are not making any mistakes. Once you can play a passage 5 times with no mistake, you can inch up the tempo a bit. Keep doing that until you reach the desired tempo. I think it’s actually harder to play something slower than faster, because all your mistakes are obvious (which is exactly why slow is a good practice tool).
The actual BPM beats per minute you would set your metronome on to reach your target tempo is also a bit misleading. If you set your quarter notes as one click, you might be set at 50 BPM on your metronome. But if you’ve got the same tempo but have some tricky dotted quarter notes to master, you might want to set your eighth notes as one click each…. so your metronome would be set at 100 BPM but your tempo would still be 50 (You just have twice the clicks per measure)
There are some really cool metronome apps out there that have a tap feature. You just tap the screen to the beat and it automatically sets the metronome speed to match your tapped in tempo.
Hope some of that helps!! C