It was originally going to work with percents, but considering that all the entities respawn when Blocky Man dies, it didn't exactly seem like a good plan.
Next up, I have to rework the map editor so it can be used for custom levels.
The map editor will work as follows. It will start you off with a small sized map, and an absolute maximum amount of tiles you can have. The current size of your map will be shown as lines on the map, and if you place a tile outside of that line, the map will extend to where you placed the tile. Actually, the hard part is that if that tile is deleted, the map size will have to shrink back again.
All maps will be saved in a folder set aside for custom maps. When you go to play a custom map, the game will read in the contents of the folder, and give a list of all the custom maps that can be played.
I also might add a test button so the map can be play-tested straight from the editor, but that seems a little much.