So her dose would be 285mg twice a day.
Since you have 875mg tablets (are they solid or capsules?)
I'd crush a tablet (or empty a capsule) into 1teaspoon coconut oil, mix until well incorporated. Divide this teaspoon into 3 equal parts, roll each one into a ball or pill shape. Put in the freezer and let harden. You now have three, +/_ 291mg Amoxicillin "pills" ready to give. This is what I'd do, 291 is close enough to 285, you have a bit of wiggle with dosing. (Of course you can make more if you wish, I'd just keep them frozen so they are easy to administer just a like a pill).
Does this make sense?