The short answer is “If you don’t have to wear all your clothes on the coldest night, your sleeping bag is too heavy.”
I tour with a 40F Big Agnes down bag. Like most BA sleeping bag it has no insulation on the bottom side but relies totally on the mattress for warmth and protection from the ground. Layering clothing I’ve managed to sleep more or less comfortably in sub freezing temperatures, 30 to 29 deg F. The layers: 2 pr riding socks + Showers Pass WPB socks, liner shorts plus leg warmers + Zoic MTB convertible pants/short with legs zipped on and cuffs tucked into the SP socks + SP wpb pants with the cuff velcroed snuggly at the ankles, a silk turtleneck + a summer weight ls jersey + a winter weight ls jersey + a lightweight down jacket (really helps) + on the most chilling night I dawn my SP wpb jacket, my hands are gloved with a pair of fleece gloves + SP wpb over gloves, and finally and most importantly my balaclava + my SP wpb rain hood cover my head (The sleeping bag has no hood.). The head situation need some work. I do use a silk liner for comfort and cleanliness. The liner adds 1 or 2 degrees F of warmth at most.

I use a Cascade Design Thermarest Trekker R-3.2. I’ve never felt cold or losing body heat no matter the night temperature or worse moisture in the soil. I previously had the original Thermarest (R-2.6 I think) I can’t says there was a noticeable difference in my comfort between the two. Both are many times warmer than the Big Agnes uninsulated air mattress I first use.

If I planned to spend most night in temperature in the low 30s or colder I’d get a warmer sleeping bag.