Abacus EZ-Landmark question

I downloaded the demo of this program last night and am quite impressed.I have a question, however, regarding the relevance of the altitude parameter for landmarks contained in the database. According to the documentation, the recommended elevation value for a given landmark is the highest point of that landmark (presumably in MSL but the documentation does not make this clear). I noticed, however, that quite a few landmarks contained in the databases list altitudes of 0. I assume this is because elevation data for that particular landmark is unknown.I intend to add a very significant number of landmarks (at least 500) to the database for which the elevations are unknown, or at least partially known. To obtain even an approximate elevation for each and every one of them would be extremely time consuming and impractical. What then, is the ramafication of using altitude "0" for these landmarks? What happens, for instance, if a landmark is a house located on a hill 3,000 ft MSL?Also, does using altitude "0" effect the calculation of the landmark distance in any way. For example, if you are 6,000 feet out in altitdue, does the distance then indicate you are more than 10 nm away from that landmark when you are infact 9 nm away?