I always lay mine down to hatch as long as the air cells look good and aren't detached or something. Congrats on your first hatch. You must be excited.
Welcome! Good luck with your hatch, not long now before we should all have chicks! and of course I'm turning around and setting more right away for the Easter hatchalong, lol.
Well, by the time they're as far along as ours are, they are actually producing some of their own heat, and they can survive heat loss much better than when they are younger. Metzer Farms has a good article on it, let me see if I can find it. My hatch was pretty good, only lost one egg.
Edit...
I bought a really powerful little flashlight at Walmart about a year and half ago that is over 200 lumens, and that thing lights up anything, even dark marans eggs. I wish I could remember the brand because it's really nice. I know it was $20 or less.
I already did, lol. I'm terrible about it and I candle a lot, but with my candler I can usually see development by day three, sometimes day two with small white eggs. So far at least nine of twelve are coming along :)