I hate getting cold, so I use a marmot helium 15. I can always unzip it or use as a quilt. I'd personally not go over 15 in the mountains here in Colorado.
I use a WM 30* frequently and take a Kifaru Woobie as an added blanket for sudden temp drops. The Woobie is nice while waiting for your morning coffee or evening liquor. LOL
are you a cold or warm sleeper? I use a 30 degree Feathered friends and I could probably get by with a 40 degree Feathered friends or Westerm Mountaineering.
WM has a 20* bag that I find to be a very accurate rating. I too just unzip it and use as a quilt when it's a little warmer during the early season. It may be in that sweet spot you're looking for.