Following up on my previous post.
First: I'm typing this on my cell phone, so long complicated missives are a bit difficult.
Second: I model N scale, so any numbers bandied about need to be multiplied by 2 so that HOers can relate.
Third: the OP was about HO scale curves, but since the OPer seems to have dropped out and since Atlantic Central wanted to expand the thread to a broader discussion, I'll bite.
Fourth: specific acknowledgement that opinions may vary.
The absolute minimum radius on my layout is 18", and those occur mainly in the yards due to the Peco medium turnouts (estimated to be #6). Most of the curves are in the 24" to 30" range. Several are in the 36" to 48" range. A couple are in the 60" to 90" range. I have one long, broad curve that is 72" radius, but it runs exactly parallel with the fascia. So, do I get points for such a broad curve? Or, do I lose points because it follows the fascia?
My train room is 24' by 25' and the layout footprint is about 18' by 25'. I look at it as the world's largest diorama, with a few kenetic elements tossed in here and there for fun. My main design criteria was to create scenic vistas, and if spectators stand in certain locations around the layout and look in particular directions, they'd see the vistas I intended to create. I will do my best to try to make those scenes as un-boring as I can. Here is a sketch of the layout with the vantage points I wanted to include. There are two #4 viewpoints (Dang!)