I could only do it by inserting a point in sketch-in-view mode, mid-line constraining that point, then ordinate dimensioning to it. The point is a bit 'stand out' and I couldn't find a way to hide it.
Had another go - neater by sketching in a small line (circled in image), mid-line constrain one end of that to the line ins question, then ordinate dim.
I could only do it by inserting a point in sketch-in-view mode, mid-line constraining that point, then ordinate dimensioning to it. The point is a bit 'stand out' and I couldn't find a way to hide it.