Why fight it?
You were living in a house owned by both of you.
OR should be a reasonable amount though, less than market rate I think. The reason for this is that it was not by choice you were living there alone would not have rented such an expensive place if you had a choice. If she rented, perhaps use the value of her rent - this would be fair.
Also, the OR amount should possibly be reduced by her marginal tax rate. It may not be fair that you rent her portion of the house, but she does not pay tax on that as it was not income that was reported. Not sure about that one, especially considering that she probably was renting at the time and had to pay for that in post tax dollars.
Also, any money spent on property taxes, up keep, etc should be split too.