Do you NEED -35F? If you do I would buy a proper brand sleeping bag, and not mid tier. -35f is nothing to mess around with. And I would get a mummy bag in that case, and I do not like mummy bags.
Teton themselves are a decent brand, but like most mid tier brands their advertised rating is not for comfort but survival, and usually a\~10 degree fudge factor. Similar to "on paper vs reality". Square bags also do not keep in heat well compared to a mummy.
If you need sub freezing I would do more research.
Things like how they insulate the zipper can make of break a decent night of sleep. I have a +20 degree Teton synthetic rectangle, it's the XXL size. And although it's super nice for the price and keeps me warm I noticed around 38-40 degrees if there's any wind it gets into the sleeping bag through the zipper. Not saying this will do the same, just an example.
Ive used their 20 degree bags in mid 30s to high 20s before and it did well enough, I was still able to comfortably sleep even though it started to push that comfort rating.
I'd trust the teton brand to be accurate for their temp guidelines, heavy af though.
I have a Teton 20° bag and I put a down puffer blanket in the bottom, under my legs, and around and over my feet, and overtop my legs, *inside* the sleeping bag. Then I wrap a second puffer blanket overtop on the outside. This, good wool socks, and sleeping in smartwool base layers, and I was toasty down into the low 20s.
Mayday-J
