While I second, that it makes sense to progress the shorter distances, as they are important when someone wants to get really fast, I feel like your formula ist was oft the charts.
The assumption that most people are running marathons at 33 - 35 x 1500 time is perhaps indicator of optimal Marathon time, but for sure not normal. It is a rough guess for elites.
E.g.: Most 2:20 runners run master 1500 than 4:00 - 4:14.
Most 2:30 runners run faster than 4:17 - 4:32
Most 3:00 runners run faster than 5:08 - 5:27.
So for most people the Marathon ceiling is way slower than 33 - 35 x 1500 imo. But for very fast runners I like the formula!