Xarelto isn’t like coumadin (warfarin) in the need for ongoing testing to check that you’re getting a therapeutic dose. The standard dose is the standard dose that is used in this setting.
I also don’t know of any data about ground flaxseed and flaxseed oil interacting with Xarelto. I’m afraid you’re likely asking questions that have no answer. You could ask your pharmacist, but frankly I’d be very surprised if there are data out there to address your question. There just aren’t reported studies done on potential interactions of every medication with every other ingestible substance.