18% Neutral Gray Paint?

[silveror0]

Now, OP, aren't you glad you asked?

 

[wiltw]

I once made a checkerboard pattern of alternating black squares and white spaces, with an even number of each on a page. Printed it on my printer on photo paper and I took a reflected light reading of that and it DID NOT MATCH a reflected light reading from an 18% tonality card in the same light.

That's because a checkerboard is not 18% reflectance; its 50%. For 18% reflectance, you'd need 100 squares, with only 18 of them white!

I am so embarrassed at how brain dead I was in performing the first (errant) test! Not a 'brainfart', it was a stinking pile!
I just regenerated a new pattern whereby I removed 18.5% of the (black) area leaving white background, and made a print on plain bond paper. I then measured

• the ambient (artificial) light with my Minolta Autometer Vf incident meter and got ISO 400 1/40 f/2.8 +0.1EV.
• measuring the printed pattern with my Canon 7DII what was measured was ISO 400 1/40 f/2.8