Draw a circle Rad 100mm.
circumf is 2 x pye x R
thats 2 x 22/7 x 100 = 628.5714
slice circle in half, circumf is now this / 2 = 314.2857
Curve Length dim gives 314.16
lets try a 500mm rad circle cut in half, should be 1571.428 per the maths but Curve length command gives 1570.8