You have it correct, but you are looking at a very narrow range of ISP.
The BepiColombo mission to mercury uses an ion thruster with a ISP of 4,200 seconds(41,202 M/s). The same formula yields a mass ratio of 11.325.
The NEXIS ion thruster can achieve 8000 seconds (78,200 m/s)
The European DS4G innovative thruster manages 19,300 seconds (188,200 m/s) ...