Depends upon which way the wind is blowing. If it is a direction field or stables are exposed to then for wind-chill, if they are sheltered then actual temperature.
I cast an eye over the weather forecast so that I know what they are predicting and then when I'm outside with the horses I look at them. Then taking into account how warm/just right/cold they are and what the weather should do I adjust rugs as required.
You must rug for the individual horse and the situation in its field / stable .
Horses vary enormously Sky is a very hot horse and you need to rug him minimally even in winter you can leave him unrugged in his stable during the day ( he’s clipped ) in the field he dislikes anything over 100 grammes even when it’s cold and wet but he’s never bothered
H is the opposite he feels the cold especially when tired and likes to be rugged .
They are both ID’s both always prone to getting fat .
many many factors are relevant when rugging it makes no sense to kept thin horses cool its wasting calories .
Fat horses can be keept cool even cold to help weight loss even though H likes to be warm he’s finished work now for his yearly break so it’s no rug during the day when in and tomorrow his out door rug will be lighter I will give him a week and get his night rug down to 100 Grams , it’s 200 atm.
The other thing you need to do is know your rugs not all rugs are equally warm if you judge them on there grams some brands are much warmer than others and all rugs get less warm as they age because the hollow fill collapses over time .
I rug by feel of the individual horse. My old boy Domino, a good doer, could be freshly clipped and still feel toasty under a medium weight in the middle of winter; minus 5ish. Harry on the other hand has to have a heavy weight rug with medium weight liner on at the same temperature because he feels the cold and drops weight easily. Haylage was/is always available.
I always go by feel, especially as I haven't clipped this year and she is a lot warmer than I anticipated! I am very lucky that I not only have a fantastic YO which does late night checks at 11pm and I mean a real check over each horse rather than just turn a light on and off so she lets us know if they were ok or too warm etc.
J has only been in a 0g and that's when its very cold wind (no shelter). He has a neck on when its persistent rain. He has nothing on if the sun is out.
Well mine are clipped out just body legs on and they had 200g rugs on today, I felt cold so I went and felt them and I alwaysfeel them in the wither area and on top of there bum, both of them felt a bit cold there so I put a heavier rug on with a neck, glad I did as it started chucking it down and it felt freezing, I have found this year they have seemed colder I think it's the constant wet and windy weather they have had there heavy rugs on quite a bit this year.