Stick to using the temperature range, as this has to be measured to a precise standard.
Even that doesn't work in the 'real' world, as such.
The test is on a dummy in a lab. at a constant temp. that stays 'still'.
Doesn't account for wind, rain, tent, sleeping mat used, temp. rising/ falling during, condensation, tossing and turning, etc. etc.
Nor wheather you sleep 'hot' or 'cold'. IE. take me, I sleep 'very hot', I know that I can sleep comfortably, in shorts, at -5c, in a bag that is rated +5c. If I add baselayers I can use to @ -10c. By comfortably I mean I won't be cold, sometimes to warm.
But then I'm not 'normal'. Everyone sleeps different. As said it can be a 'minefield' in finding what bag suits you.