You need more miles :-)
I know Joel is the author of a highly praised book on power/watt training but this algorithm is a worthless generalization for a competitive cyclist. And for the non-competitive cyclist, it has no merit or useful value other than more BS to spew.
(even less worth while than the charts that calculate your maximum HR based on your age)
I must be channeling the "old" umd but I think Joel is reaching to fill space on a blog.