Creating constructions with command blocks isnt quite easy. Often you use relative coordinates to get away from hard coded coordinates. But calculating them is a lot of hard work. Thats where this plugin comes in.
Features:
Very lightweight
Simple usage
Plug n Play
No worldedit needed
Selection tool changeable
How to use:
After installing the plugin, grab a slime ball and do a left click on your parent block (Your command block, where your command gets executed). After that do a right click on your final destination. Then type in /getrelative. Now your relative coordinates get displayed in the chat.