How I "measured" the "about ISO 10":
I adjusted the enlarger light to approximate sunlight (added some cyan+magenta) so the comparison would be "fair".
Then I exposed my step wedge film (I have made it before on BW film) to Agfapan APX100 at a specific exposure and processed in XTOL 1+2 for 3 mins.
Then I opened lens three stops and exposed my film and processed it in the same way.
After processing, I noticed that the lowest density wedges on my film and APX100 match quite closely. And, because I opened lens three stops, it's ISO 12,5. Very precise .