This calibration takes into account all your printer profile's first layer settings, giving a perfect Z-Offset. Use an opaque filament.

1. Use your printer's initial calibration feature if you have any

2. Print “Z-Offset Calibration.stl” and let it cool. (Use the tab at the top to lever it off the bed)

3. Examine the print:

   • If the lines are not joined, lower the Z-Offset (Too High)

   • If the surface is rough, raise the  Z-Offset (Too Low)

   • If the print is see-through, raise the Z-Offset (Much Too Low)

4. Repeat until the lines of the print are joined and smooth.

If you are really anal like me, you can use a set of calipers to make sure the print is 0.25 mm. If you get to 0.25 mm and the lines aren't joined, then increase your first layer flow. Repeat until the lines of the print are joined and smooth.

Use the First Layer Flow Calibration to tune in your layer height/flow/bed adhesion. Printing these shapes with Archimedian Cords lets the infill print over itself at one point. 

Shown here:

Like with the newer flow test (Orca Slicer 2.3.1+), this causes a deformation of the earlier applied line nearest the new line.  Once you have a sample that is good, use a set of calipers to measure from the bed to the top of the layer. You can tune it till you get your exact layer height with a proper surface.

Bear in mind that the closer to the bed you are with more flow, the better your bed adhesion will be.

I found my best was at .85 flow rate for my material.

Feeler Gauge Method:

Use the feelers to set your nozzle height, and then use this test to set your flow rate.

Good Luck, and Happy Printing!