I haven't read the article so I could be full of shit, but essentially:

If you have the list of desired things ready, there should be an ETA on the work time necessary for each desired thing as well as confidence on that estimate. Confidence varies with past data and expected competence, e.g. how easily you believe you can debug the feature if you begin to draft it. Or such. Then you have a set of estimates for each implementable feature.

Then you put in time on that feature over the day tracked by some passive monitoring program like ManictTime or something like it.

The ratio of time spent on work that counted towards your features over the work that didn't is your productivity metric. As time goes on your confidence is calibrated in your feature-implementation work time estimates.