Jeroen,
I've got 1 last question for the moment. I also prefer a setup as simple as possible, so last time I cut the led in 4 pieces and this time I was hoping to not cut it (need to make a choice then about the corners, I know). But I'm afraid the current might not be strong enough to reach the last leds. Is that a matter of a strong enough psu or can I connect 5v to start AND the end of the strip? I don't understand how that could work having 5v on both ends (and I thought the ledstrip has a 1-way direction but maybe that's only for the data line?). This is what I understand you did:
soldering is very minimalistic and because the ledstrip is only shortened 20leds chance of mallfunction is very low. where i made the cut i roughen up the solder on +5v and GND and connected this also on PSU.
so powered from both sides.
I am not an electrician so I don't understand how 5v to both ends would work. For me that sounds like no voltage difference so no current.