Having just loaned out a R-5 Thermarest 60cm x190cm full winter ('Orbit' for MEC) pad plus an 18mm foam pad, a wearable Wallcreeper primaloft overbag and a Kluane -30C bag, to be combined with my buddy's -5C inner summer bag, for lows in the -40C range, I still wonder if it was enough...I hope Luc eats lots of chocolate cookies before bed time!
North of 60 where the 'summer' can be as cool as -12C if it is a clear night in August, I pull for the full-length Women's T-rest Prolite 4, I bought one after years of looking for something as warm as a T-rest LE. As long as they are used inside a tent or on a ground cloth, T-rests wear like iron. Pack a foam 'sitz' fanny pad for sitting on rocks with. I am amazed how T-rests have cored-out foam, mummy shape, etc as the Women's Prolite 4 is 750grammes for an R-4 pad, whereas a 2003 LE is 1200 grammes for the same insulation...
I looked at velcro-ing 2 Torsolites together with an existing T-rest kayak seat pad to make a 3 piece modular sleep pad, but it had 3 cold spots in it where the pads came together...so I went for the pink Prolite 4!
So, seen from the North, pack a full-length pad!