I’ve been wanting to do this so I figured I would take a stab at it.
This is a photo full frame, no cropping, of a ruler, each notch is a 1/64 of an inch, tried to get a sub 1mm ruler but I don’t have one.So, this shows about 30/64 = 0.469’s of an inch across the sensor, or 0.469 * 25.4 = 11.91 or 12mm
The D700 sensor is 36 x 23.9 mm. so the ratio of 36mm : 12mm is ===== 3:1 or 3X magnification
This is using my 150mm Sigma lens with a Sigma 2x converter and a 36mm Kenko extension tube. I could get more with the 60mm Nikon Macro if I still had one.