Here's the model I'm working on. I need to measure the distance from the midpoint of the upper triangular piece to the midpoint of the lower one. I've tried using Smart Dimension like I did on the other measurements (which are all from endpoints), but even though it tells me I'm clicking on the midpoint, the smart dimensions snap to the bottom of the top piece and the top of the bottom piece. I can't find a way to measure from mid-point to mid point, and I need that because it's a critical measurement.

Any suggestions?

So I tried that with horizontal construction lines tied to the midpoints of the two parts. I can do an angular smart dimension between the two construction lines (holding shift while dimensioning), but then when I move one of the parts, the construction lines remain locked in space. That is, they're not moving with the parts. it's not possible to do a two-entity dimension on anything other than an endpoint, it seems.

I can also do a midpoint-midpoint dimension with the vertical (non-smart) dimension tool, but again, that's fixed in space, so moving the part doesn't update the measurement. Since I need to tweak the positioning of this to fit a real-world space based on those midpoints, I'm not sure what the best way to do this is, short of a bunch of back and forth trial and error.

If you are just measuring, you can use the Tape Measure or Verify
to measure.
Larry