OK, so far, so good... but here's where I'm stuck.
I need to know the distance from the vertex at the bottom of the item to the terrain (the red dashed line), EVEN IF there is no vertex directly there (which there isn't, in my case.)
I basically need to "project" the vertex onto the terrain "tile" below, then I could determine its Y-value.
I can't think of any booleans, knifes, or plugins that would work, and reveal that distance.. do you have any ideas?