Sounds like a good idea, we will work on adding this! For now, I can tell you that we try to download more data than less data, to be sure you have everything you need for a place. We are able to do this because our data is already less than half the size of most other offline map apps. Cities usually include extra area around the city to cover suburbs, etc. Region and country downloads are more straight forward because you can see the region and country border lines on the map and know that everything within those borders will be downloaded. Also if you download a place, for example the USA, it will include discontinuous regions such as Alaska and Hawaii, usually even minor outlying islands.