I'm just like mxgirl. All of my shoes are a 5.5-6 in boys/mens, and a 7.5-8 in women's (mostly 7.5), but I first ordered a 6 in the SG10, and it was a little too tight in my toes/on top of my foot, so I actually sent them back and went with a mens 7. They are a tad big (I can lift my heel a little bit while they are on), but I've had them for 3+ years now and put lots of hours on them, and having them a little big has never been an issue. I can spend a weekend in them riding and walking and they are as comfortable as can be.
If you wear a women's size 8.5, you are probably borderline between the 7-8 in the SG10. Do you have a shop nearby where you could try them on first? Or maybe ask some riders at the local track? Or if you order them without trying, just double check the return/exchange policy first!