I've had lots of nights I'd wake up shivering when I was sleeping in the fire station (thermometer at 68, me in my shorts and Tshirt).
Anything much warmer than 68F then I might be inclined to switch on a fan to keep cool enough to get some sleep.
Does anyone know the Tog rating for a Poncho liner compared to a mylar space blanket. Knowing this should give a guide to usability temperature ratings. Comparing the rating to real world data shown here will give some indication of the poncho liners ability to keep someone warm.
http://www.mammut.ch/images/Mammut_Sleep_well_pt1_E.pdfFor example you would need around 9 Tog for comfort at 32F.
I suspect that the Poncho liner doesn't have more than 2-3 Tog i.e the limit of comfort would be around 65-68F, just as you have described. It certainly puts the thermal abilities of the Mylar blanket in to perspective. i.e. virtual no capability.
Even using some
Bivi Jacket ( approx 800 grams) and trousers (400grams) with a Tog Rating around 5-6 I probably wouldn't want to sleep in them in temperatures less than 8-10C (46-50F) and an extreme temperature (just high enough not to get hypothermic) just above freezing.