Today I have Ritter on my calendar a 4.4 threshold with a TSS of 81 and a IF of 0.90 and 704 calories when looking at some of the alternates with slightly higher PL (eg Bear Creek 4.8 or Woodworth 4.7) some have lower or equal TSS, IF and calories
I don’t think that it’s a simple “X seconds at Y Watts” type calculation in the same way as TSS & IF but is a bit more nuanced than that. There was some discussion about PLs in the original podcast when AT was introduced but can’t remember if there’s been any others delving into them.
IME how you find a particular workout also depends on your personal strengths and weaknesses: length of effort vs recovery time and so on. The two workouts you’ve highlighted are quite different - short sprint efforts aren’t really the same as sustained overs for example and a 12min over-under interval is going to be harder than 8mins just under threshold with a few sprints thrown in.
The classification system isn’t perfect and there have been various oddities highlighted since PLs went live.
The example on the podcast is that 4x10 @ threshold and 1x40 have the same TSS but are very different workouts. Looking at your example the extended time over threshold give the workouts a higher progression level. If you spike over FTP for 20 seconds on the road you’d hardly notice it but two minutes would be much more noticeable.
The higher progression workout has a longer rest which alters the IF & TSS of the ride but the “On” bits of the workout are much more challenging.