That's correct   Though only in this case because the target block reward time is in minutes, more generally it'd be  (**) / ( * ). And keep in mind that it's just an extreme simplification to get a rough expectation. After all, the divided by portion would only be true if averaged over an extremely long time with a constant network MH/s. And neither of those conditions are true in this case - we're talking about a relatively small sample size and network MH/s is constantly changing. But it still works adequately for a rough estimate.