@sgemmen Thanks for this, finally got around to setting it up. It wasn't working at first but realised I needed to add the two helpers (boolean and number) for the targets of the service.

Now I need to work on using it to adjust all the lights in a room while keeping the relative brightness of them the same.

EDIT: Found this add-on which seems to work - https://community.home-assistant.io/t/relative-brightness-light-group/695450 - brightness changes are a bit jerky (not sure if this related to flic, the add-on, or something else) but it does the job for now.