Don't waste you time by trying to get an accurate count, the "other" information is designed as simply an estimate. Also, please remember that 256GB of storage is not that much to begin with and if you need more then either return the iMac you have (assuming it's not over 2 weeks old) and buy one with 1TB or more of storage. Another option is buy yourself a high quality EHD and begin moving libraries such as your photos, music and movies to it. Another third option is to begin using iCloud and store larger libraries there.
All of these options are much better than trying to stuff a size 10 foot into a size 8 shoe.