I’m similar to Scribbler with my older horse, though mine live out 24/7. 10C with rain gets the rainsheet. There does come a point in the fall when he just keeps the rainsheet on all the time so I can have a cleaner horse to ride. By the time we get to highs around 0C I have done his modified trace clip and he’ll get his 200g at that point.
My younger horse gets a rainsheet at 15C with rain. He gets his 150g quilt underneath as soon as he shows me his signs of getting cold. He also gets a modified trace clip, but his midweight quilt is 280g. The next step is a 200g neck rug when daytime highs are in the minus teens Celcius, then a 340g quilt when lows get around -25C. I let him tell me when he got cold the night before and note what the weather was like for future reference. Wind chill affects the blanket weight decision quite a lot where we are.