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Hi Graham,
You're mostly correct in your process.
Essentially the light should be as " white " as you can make it. That way it will have an even effect over all channels. This is obviously hard to get perfect as LED lights vs Torch light vs Computer screens all give off a different tone. A white screen on the laptop though is a pretty good white so good choice there.
The flat needs to be taken at the same ISO / gain as the image to which its being applied and also the same focus. The idea behind this is that a flat is designed to compensate for vignetting and imperfections in the image train. If the focus changes, so does the focus on the imperfections. If the ISO changes same deal.
Lastly and the bit that is not quite correct although you still may have captured by accident is you need to allow the camera to decide the exposure length to get the correct level of exposure. Too long and it I'll wash out some imperfections, too short and it won't have a strong enough effect. Set your camera to AV and take the shots. Don't set to full auto otherwise your camera will change the ISO also.
I hope this helps.
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