I'm not convinced by the multiplication factor where there's a fixed ceiling.
I use a spreadsheet with my own calculation (involving metres climbed, average gradient and so on). I tried various constants before arriving at something which churned out meaningful, to me, classifications for my rides.
For example,
22km lunchtime ride = 1.4
100km Faccome Haul grimpeur = 16.2
322km Andover to Manchester = 32.2
100km Denmead Audax = 8.0
200km New Forest Excursion = 13.4
The actual numbers are not meaningful to anyone else unless they agree on my numbers but it means I can plot a ride beforehand and get a feel for how tough it will feel FOR ME.
So you can see that the 100km Faccombe Haul comes out as slightly harder than the 200km New Forest ride. Let's assume a bit of 'play' either way and I reckon that's about right. The New Forest Excursion is very flat and the Faccombe Haul isn't. The average gradient of Faccombe Haul weights the classification significantly.
If a ride comes out >20 then I can assume it's going to be quite strenuous, <10 and I won't need to prepare for it and so on.
I can't put a limit on a ride (such as 10= you can't do it) because there are too many factors, such as speed, terrain, weather, food consupmtion and, at longer distances, psychological considerations (can sometimes make you THINK you can't go further when you actually could, physically).
Maybe the 1.7 factor works for a constant speed and constant terrain but slowing down by just 1mph can sometimes add several more hours cycling.