Taxpayer jointly owns home w/ a non-spouse.  Office space is used exclusively by taxpayer.  For total area, should the remaining part of the home square footage be divided by 50% and then add the office space square footage?  Dividing any utilities in 1/2 assuming that is how they paid.  

Is there a better way to determine percentage of office space versus the one proposed above?  They are an unmarried couple.  Perhaps just treat it as you would for a married couple?