Ive had my 40* set down to mid to upper 30's and fine with a light baselayer on and socks etc. Low 30's with same stuff plus my down sleeves on my feet and my UL down T on . Using a thin 1/8" pad for my legs. Now a 20* UQ and a 40 Tq is a fun combonIve had low 30's no problem with a mid weight base layer and toboggan an 1/8" pad for my legs.
Probably could get that setup a bit lower.
A good pitch and location can make or break ya too. Stay on that Lee side....etc
Sticking feathers up your butt does not make you a chicken!