|Upside-down view of overhang not sufficiently supported|
Put piece of paper under nozzle
Move down Z by 0.1 at a time, checking if the paper can still be moved. It will be held slightly at some point, with 0.1 more it will be held fairly firmly.
Set Z offset to 1-#moves.
Spray bed with hairspray (still need to figure out if cold or hot bed is better when spraying, whether to dry off the spray, and whether to do multiple layers)
After talking with +Richard Jørgensen about his setup, I'm quite tempted to add a Raspberry Pi to control mine. Having to tether my main laptop for printing is not very handy (the SD card reader appears to be broken). Fortunately, nophead has already shown how.