That calculation was based on Manchester's population of 553,000. However let's look at the total population of Devon and Cornwall (1.4 million) vs the metro area of Manchester (2.8 million). That may look like it's much closer, however 0.5tph only goes to Exeter, and 0.5tph goes beyond Plymouth.
As such you're likely looking at an average population of maybe 0.75 million vs 2.8 million, so that's still 11.5 vs 36 coaches.
However, Exeter has trains via Bristol which leave later and arrive sooner, which I hadn't previously included for in the calculation, which Manchester doesn't have.
Nor have I allowed for the fact that some of the population of Devon is East of Exeter and on the WofE line which I had already removed from the coach calculation which I had already done on the previous one, but not accounted for above in the population numbers.
East Devon DC has a population of 144,000, even if you only account for 1/2 of that population going east rather than west to go to London then you end up with about the same 4.2 ratio between Exeter and Manchester.