I think 20 degrees warmer than the bag rating is generally the comfort temp, though it varies by person/layering/pad etc.
I have a thermarest xtherm and a marmot helium that is 11 years old, I run pretty cold but with layering have been comfortable sleeping down to 15 degrees maybe a bit lower… the decent sized creek we were camping near froze over during the night so whatever temp that is… merino wool + synthetic insulation seems to work best for me.
I’m in the market for a new bag though and I’m probably going with a WM… I think in the 0-5 degree range, will then use the marmot mostly for early season trips.
Also I’ve been really impressed by the REI bags, the magma is pretty great at that price point.