It’s really just 10(-24/93*150) ≈ 10-39.
If you want McLatchie’s 10-53 you need to do 10(-24/93*202). But then where you get 202 from is a genuine mystery.
But I really have to emphasize once again that I consider all of this crap about what fraction of sequence space corresponds to a specific structure-function relationship (as Ann Gauger would say; “that structure with that function”) to be a red herring when it comes to evolution.
Why? Because evolution isn’t a theory that says any and all functions evolved by directly selecting for them with blind random sampling into protein sequence space. Many, many proteins have evolved by duplication and divergence.
Proteins with particular structures and functions can evolve from proteins with similar or even other structures and functions (serine beta-lactamases, the family TEM-1 belongs to, evolved from DD-transpeptidases), so the fraction alone tells us nothing about whether the function could evolve. And more complex structures and functions can evolve from simpler ones. In fact that very pathway was exploited to evolve beta-lactamase activity in catalytic antibodies (binding first, which is easier, then catalysis from binding). So there’s actually good evidence that there is a lot of overlap in function-space, and that selection can drive sequences from more frequent (easier-to-discover functions), into less frequent, harder-to-discover functions.
That’s why, when you claim the numbers represent a problem for evolution I ask you to show your work.