In my experience, Great Circle Mapper has been +/- 5 or so miles on what Frontier has awarded on flown. However, one thing that is confusing is around if you would get the sum of the flights or not. Looks like the flight # does not change even with the stop in ATL so you would end up getting just the LGA-DEN miles. From their terms and conditions: Members flying without a change in flight number between origin and final destination will earn the actual miles computed between the origin and destination city without including the stop over.

If you had a change in flight number you would get both amounts.