the lens + (Nikon BR-2A) + (F->M42) + (M42-M42 Helicoid) + (M42->E) + a7
(this is almost same as the lens reversely mounted on any Nikon camera)

You can take out M42 helicoid, in this case the magnification is about 0.84X,
if you replace bigger helicoid, the magnification goes up more than life size.
With this helicoid (17~32mm) without extended, the mag is ~0.92X and fully
extended case the mag is ~1.33X. I have not tested with the other helicoids.