Just like the line tool, you should be able to manually type a radius to use for the fillet arc tools, currently this is not possible.
In addition, with radius unchecked, there should be an option for rounding the radius. So when the fillet is about right, ticking this checkbox would round the fillet to the nearest whole number, i.e. 7479 could become 7500mm or 7475mm depending on the rounding increment (100, or 25 respectively). That would make the tool more pleasant and fast to use.