My experience leading groups into the woods all winter for two winters is that everyone is alarmingly different. Different metabolisms, different amount of food eaten that night, different comfort levels, etc. Survival is probably similar, but comfort is not.
I, myself, figure out my range as the temperatures drop on my porch. I slowly reduce the amount of insulation relative to the temperature until I cannot sleep. Once I know where that point is, I spend the next 5 months choosing my sleeping bag and insulation confidently.
I would look at what you already have, and assume the WM bag is going to be exactly what it says it is. If your 30º bag is comfortable at 30º for you, a WM Summerlite will definitely be comfortable. And keep in mind that your fluctuation could be due to something as simple as whether or not you had food before bed.
Personally? I took a 20º bag and a 45º bag together down to -15ºF last winter, and I managed to sleep a night at -5º using just the 20º bag. If I owned a Summerlite, I would probably take it to 10º.
I also took a 45º Quest bag from the 1990's down to 20º... so I'm not a good example. A girl in our camping group made it past my foolproof gear shakeout with a 45º bag for a Maine trip. I had no idea. Since our spare bag got wet, I gave her mine and toughed it out. It was a 5'5" bag and I'm 6'2", and I still managed to sleep through the night in a synthetic puffy and rain gear. We had a big laugh about it in the morning when we actually read the temperature.