thank you Sadie and Smartie!
I did a tour of my house this morning with a sheet of white paper and my ruler. Gave me lots of surprising results!

So locations 1,3 and 4 are out of the question: hand with fingers spread, I can only get a fuzzy shadow when my hand is about 3 or 4" from the paper.
Location 2 is not that great either, only 4.5" away! I was shocked! the other locations in this room give me the same 'rating' or less. The pb with that room is that it's either full direct sun, or on the side walls, and as soon as I move away from the direct sun (in any direction), I get the 4" distance.

So I checked for another location, downstairs (like locations 3&4), on a coffee table in front of the south facing window. We have a porch in front of that window, so we never have any direct sun. However, as much as I always thought it was a darker area than room #2, the paper test gives me a fuzzy edge shadow at about 6-6.5" from the paper, when the paper is 30" from the radiator.
This room is about 16°C at night and 20°C during the day. A bit cooler than #2, but it seems I'd get more light out of it. There are pot lights here as well so I could turn them on at night for extra light in the winter.

I'll post pics later of my current setting.