I assume that your assignment is to develop an opinion of market value. A potential buyer will be stuck with any existing leases and rents that are in place at the time of purchase. If you are dealing with a long term lease, and actual rents are below market then, consider using actual rents. If the remaining term of the lease is short... say 6 months or less... then go ahead and use market rents. Be sure to comment that actual rents for the subject are below (or above) market rents. You could, if you want to get into it, capitalize the rent loss between actual and market rents for the remaining period of the lease and apply that in your valuation.