to save money & effort, Buy her a mediumweight standard neck rug (e.g. 100-200g) and keep that on her until the winter really hits, then throw this one other the top of it. THAT said, horses feel the cold in different ways and I know a lot of TB people who rug up to the eyeballs and their horses are not suffering for it. Just, as someone said, get a
duvet or bigger rug for when it gets colder...