Is steam lost in boiling negligible?
This depends on the time you need to bring it to boil (water evaporates long before it starts boiling), the surface area of the water, and your definition of "negligible". Best test it with your own kettle. 

My kettle needed 2.5 minutes for boiling it, and lost 11 g out of 560, not quite half an ounce. It will probably lose some more while cooling. If you use an open kettle, a very wide kettle, or heat the kettle slowly, you will lose more.