If all is right, IG designates the differences of things like varying spread to time periods. For example, from 02:00 to 15:30 for medium, 15:30 to 22:00 for high and 22:00 to 02:00 for low (low would be the 1,04 from fifi). What the time periods are should be mentioned on the same page.
Whether your proposed 0,6 is correct, depends on the trading times. Is that not under your control ? then it is obviously always a too low number. But do you trade always in the “0,6 period” then it would be correct (for EUR/USD).