Allen and Coggan suggested that 5% less of a 20 minute average is a more reasonable way to estimate FTP than a straight 60 minute effort for various reasons. They go onto explain a power profile protocol to find limiters and strengths. Perhaps that’s where you’re getting confused and I’m not aware of a Friel 30 minute test.
Anyways, I think the most important point in all this FTP modeling is we are doing it to effectively work all three systems (ATP-PCr, Glycolitic, Oxidative). Having a test that is easy to repeat yielding consistent results over time is the point. Are you improving, flat or declining? Basically, a validation that what you’re doing is working.