tjeulink, just for your information, you do not need to run your enlarger with this analyser, you can, but you don't have to.
I myself don't have my enlarger controlled by the Color Star analyser, I use the analyser as a stand alone unit.
When you do use it, you will need the darkroom lights completely off, not even safelights on. The sensitivity (readings) of the analyser will be incorrect if there is any kind of room light. You will often have light escaping from the negative stage of your enlarger, providing it isn't excessive, this will not be a problem.
My darkroom walls are painted white, even with some light escaping from the negative stage of my enlarger and being reflected in the darkroom, this has never been an issue.
I would also suggest that if you wish to do a size change of a print, either up or down, then the most accurate way is this:
First, find your perfect print exposure time, as well as the aperture on the enlarging lens. With all lights out, half pull the negative stage out and place your sensor directly underneath, switch the enlarger on. Set the sliding scale on the left of the analyser so that the readout in seconds matches your exposure time in seconds.
Replace the negative stage, switch some lights on then move your enlarger head up or down and focus the enlarger for a sharp print just by eye. Switch the lights off, half pull the negative stage out, place the sensor directly underneath and switch the enlarger on. The analyser will then display your new enlarging time down to one tenth of a second.
At this stage, critically re-focus your negative, then give it a go.
Within reason, this is super accurate. Once you have done it a few times you will be better equipped for fast size changes in the darkroom.
There is one caveat though. If you go from a smallish print size, say 100x150mm to 400x600mm you may need to add slightly more time due to the vagaries of light loss. Your new time will be very, very close, but not 100% close. I have found an increase of 1/10th of a stop more light is often needed if going up in scale by around that amount of difference. Not always, but quite often.
You may also wish to change the contrast by something like ¼ of a grade, or even by as little as 1/8th of a contrast grade change. Changing contrast grades in these small amounts can only be done by use of either a colour head or a dedicated B&W head on your enlarger.
Hope this helps.