12/8/2021 - The Nikon D500 crop sensor has the dimensions 24x16mm. This is in contrast to “full frame” cameras that have a sensor size of 35x24mm. Macro typically means that an object is projected onto the sensor in it’s full glory. For example a US quarter is about 24mm across. It photographed in macro means the quarter spans the entirety of the sensor from left to right, or in other words, the quarter fills the frame.
One evening I thought it’d be interesting to see how much magnification I can get out my kit by photographing a ruler at the highest magnification of various lens configurations and set to the closest focusing distance. The camera was on a Platypod that I would slide back and forth to get a sharp image. 
Up first was the 18-300, maxed out at 300mm the minimum focusing distance per the lens is x mm. The reading on the ruler was 64mm, which means about a 1:2.6 magnification (64/24). With the added Canon Close-up filter 50D the distance from left to right is 54mm.
Up next the Tokina 100mm f2.8 macro lens. Dialed in at the closest focusing distance the reading from left to right is 24mm, so it is true that this is a real macro lens.
Then I added the Kenko extension tubes, smallest 15mm, medium 18 and largest 20mm. With al three attached to the Tokina the image is 12mm across, meaning this is a 2:1 magnification.
Lastly the Canon 50D close-up filter shaves off another millimeter, from 12mm to 11mm means almost another 8%.  I didn’t feel very comfortable with this configuration, fearing it might rip out the connector from the camera housing. But the below table shows the rough magnifications or various setups.
Configuration                                                        Across       Magnification
Nikkor 18-300                                                       64mm               1:3
Nikkor 18-300 + 50D                                             54mm
Tokina 100mm                                                      24mm                1:1
Tokina 100mm + 50D                                            20mm
Tokina 100mm + 1st Ext tube                                 20mm
Tokina 100mm + 2nd Ext tube                                18mm 
Tokina 100mm + 3rd Ext tube                                 15mm            1.5:1
Tokina 100mm + 1st, 2nd + 3rd Ext tubes               12mm               2:1
Tokina 100mm + 1st, 2nd + 3rd Et + CUF                11mm
Thank you!
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