52M km^2 of land on our planet is habitable, after forests, shrubs, rivers and lakes [0]. Total land mass is 149M km^2. So only 1/3 of the world's land is habitable. That's actually ~20M square miles. Your habitable land estimate is already nearly 50% off according to the source I found.

The circle covers 50.2M km^2 (4000^2 * pi). I'm going to assume the 2/3 land quotes are good enough an estimate. That makes the land mass inside the circle 33.5M km^2 (2/3 * 50.2). That means that the circle contains just 22.5% (33.5M/149M) of ALL land mass.

I couldn't find any quick data source on how much land inside the circle is actually habitable. So I'll apply a few ratios:

If X% of the land in the circle is habitable:

  X = 33% --> 33% * 33.5M / 52M = 21.5%  
  X = 50% --> 50% * 33.5M / 52M = 32.2%  
  X = 75% --> 75% * 33.5M / 52M = 48.3%

Your estimations would hold iff >75% of the land in the circle is habitable.

[0]: https://ourworldindata.org/land-use