How do you choose clothing, sleep system for a given trip?

In general, I go by historical climate lows for the month rather than forecasts. I use extreme low for sleep system, using clothing to help out for close to extreme lows if encountered. In mid winter I also depend on fire and improvised shelter also, in the event of an extreme low event, but also bail out strategies. I tend to keep the trips short midwinter also, at least until I get a -25F sleeping bag. When we do get that, I am usually at home, testing what I do have in the backyard, with extra clothing and foam pad etc, to remind me just how cold and dangerous -25F is when illprepared. Fun stuff. Would be less fun in the woods.

Clothing wise I usually bring enough for Normal Daytime Lows for the month, or about 10C warmer than Extreme Lows, whichever is colder. I bring enough to stay warm while trudging slowly, sunrise to sunset, and stopping for up to an hour at a time in a sheltered area, without having to use the sleeping bag. I think a sow trudge and light rest and camp activity in a sheltered area require about the same clothing. So far I empirically figure it to be about 1oz of clothing per degF below 80F, not counting shoes and shells. For temps below 0F I will add more safety factor. I layer evenly for the worst conditions, one set, plus extra socks or two. For below 0F I also bring a set of long wool underwear as an extra. They can fit over skin layers and under mid layers for an extreme low event, or in the event of falling through ice or whatever.

My hiking is in coastal forest, rather than Alpine, although it includes some exposure on open lakes and rivers, especially for travel on ski trips. Stopping and overnights are in the woods though, with drying fires as an option in emergencies. Snow shelters are not alway feasible. I don't use down clothing, or synthetic batt, but I am considering some sort of a down or batt long coat that doubles as an overquilt for my sleeping bag. I might use my CF gortex bivy as the outer shell. Not sure of best inner shell. I would have all my other clothing under it, including light wind shell.

Shells tend to ice up below 0F, which complicates things, but where I am Maritme Coastal I still need to be prepared for wet stuff around freezing in addition to dry stuff below 0F, so I am staying away from stuff like light canvas shells. Fleece makes an interesting semi-shell over wool, with some wind shell and rain shell to back them up as needed. Wool itelf will work pretty good in shedding snow and icing over a little on the surface as it gets colder.