Silent video (added link above also).

What do you need at the end?  A solid model or a dimensioned drawing?

I don't dimension anything unless I need to make it variable geometry for parametric analysis.

You drew a circle and it had a radius of 31.14 mm At the time you drew that circle, a box popped up with that dimension. That was your chance to type in 31.2

If I accidentally lose the box with the dimension in it, I delete the curve and draw it again. I don't add a dimension to try to drive the dimension to a different value.

Start over and type in 31.2 when the box is offered. Don't go forward with the wrong dimension.