Those two wiires come from my desktop power supply currently set to 4.5v.
As to "Am I using it", answer is best given in stages...
Stage 1;
There were links between the power rails in the centre and I was just using the power from the Arduino. When connected like this the LED display only reads 244 at maximum (it reads 255 with the box of WS2812 LEDs disconnected) and if left at 244 for more than about 10 seconds, the Arduino would power down/disconnect. So...
Stage 2;
With the connectors still in place between the two power rails, I connected the power from my desktop suipply (4.5v). LEDs flash like crazy and LED display very unstable. realised I had two voltages on same line, probably not wise.
Stage 3;
removed links between power rails so power to WS2812s now entirely separate from power to Arduino and potentiometer etc. LEDs still flashing like disco ball.
Stage 4; (this morning!)
reconnected links, supplied ALL power from desktop supply, using VIn on arduino instead of usb. seems to work...display hits 255 and ws2812bs are on steady and the board isn't shutting down.
Did I finally get where I'm supposed to be?