The simple answer is yes, a fifty-fifty mix of 36mg liquid with another substance would result in 18 mg liquid.
But of course since Rita also added a small amount of flavor, her resulting mix was something less than 18 mg concentration. If she had made her flavor and glycerin mix add to up to one ml first, and then added the 1 ml of 36 mg liquid, then her result would have been exactly 18 mg. The spreadsheet, created by Jaaxx, is great if you want to be precise and know exactly what your resulting nic concentration is with varying amounts of dilution.