Now that the USFG has moved to 0.5mm girdles (yay), it’s easier to measure them with a piece of wire. Well, unless your wire is measured in square millimeters haha, but I think jewelry wire is still sold with Brown & Sharpe (American Wire Gauge in the States) designations in Australia. 24 gauge wire is 0.511mm wide. Your upper & lower limits could be 23g (0.573mm) and 26g (0.405mm), but odd sizes are not common. If you want to know when you’re “getting close” and need to stop with your cutting grit, you could use 18g (1.02mm).
B&S/AWG wire gauge doubles in width every 6 sizes, so if you remember that 18g is about 1mm, you can remember that 24g is half that, 12g is double that (2mm), etc.