What a great Sticky.
Just wanted to add a tool that I recently picked up from Lowes. I havent tested it yet but as per the original post stating the importance of a level boat to start with I think this might help and actually picked it up prior to reading this.
It is a Ball level. If you are not on level ground you can figure and mark the level horizontaly then compensate for vertical and even check the degree of angle on the rigging from port to starboard and make certain they are equal .