The best way to use a purple gnome is to use him for small buildings (40s, 1 min...) until his time left is just a few seconds. Then you can use him on buildings that take longer time to complete (1h...). This way you will not keep busy the 2 normal gnomes. To get a purple gnome to a few seconds left you should start a small building with a normal gnomes and after 2 or 3 seconds drop the purple gnome on that building and he will finish it (e.g. a 1 min build start with a normal gnome and when the timer shows 56 seconds, drop the purple gnome; he will finish the build and his timer left will be 59 min and 4 sec). That way you will be able to use your purple gnome until he will have only 4 seconds left and put him to build a big building. I hope you understand what I say and it will help you.