So, I went a little crazy and ordered a metric butt-ton of various filament samples. When I came to the end of my most recent spool of Hatchbox PLA, I started working through these samples, and I discovered something that you'd probably find in the International Journal of DUH! but it was interesting to me. I'd gotten the Z-offset dialed in a couple spools ago, so I felt no need to adjust it for any of these samples, and they all printed about as well as the Hatchbox I've been using. But, when I got to the "Transparent Yellow PLA/PHA", it looked like there wasn't anything getting extruded for the first two layers at all. Or, at least, I sure as heck couldn't see anything. So, I just let it finish, and yes, it turned out ok. Well, light color and "transparent", being laid down on Black BuildTak, I guess that isn't terribly surprising. Next came some white, and the contrast (in every sense of the word) was amazing. Yes, from the very first line on the model, I could clearly see that the first layer was setting down with the exact right height and squish as I'd intended all along. I think from now on, when I have to reset the Z, I'm going to use white filament.