Some horses run warmer than others like people do where one person is okay in a t-shirt someone else needs a light jacket while someone else will need a bigger, bulkier jacket. It's individualized towards the horse but I'd definitely follow abbeys baseline
If the horse is clipped you can blanket up to 54 degrees. If not clipped I blanket at 40-30 light blanket, 30-20 medium weight, and 20 below heavyweight. It also depends on how thick a coat your horse has. Hope this helped!
It depends on what the horses are used too. In colder climates, our horses are used to it being around 7 degrees Celsius at the very warmest before we blanket. In warmer places, horses may be more shocked by cold temperatures, in which case you would want to blanket when it's still not too cold.
You also want to take into account whether or not your horses are "good at growing winter coats", in which case they wouldn't be in as much need of a blanket. Or maybe if you wanted to blanket as a way of preventing them from growing a long, thick coat.