By default the color legend is set to the data range, so if it is set to -5 to 0.9 then it means that the maximum distance found was 5mm. Note that closest distance measurement is not symmetric, so if you get an unexpected result it might be due to that you switched up the source and target models.
Make sure you use the latest Slicer Preview Release. It should look something like this:
If you want to change the displayed range then change Data scalar range (auto) to Manual and set the range that you prefer.