I think their definition of Net Revenue Retention (at least my understanding) is wrong as it includes growth from net-new MRR. This confounds retention with growth and isn't really helpful to answer what you really want to know: where is the revenue coming from? Retention IMU should be Past MRR plus upgrades less downgrades less churn (divided by past MRR). It explicitly does not include new customers.
Some of the other metrics are not the priority ones we look at either; your core measures really depend on if you looking for profitability or growth.
Regarding the casual app/graphs, the graphs are cool but they feel very sluggish and slow to load. It takes 15 seconds to load the page + the top two graphs on a 250Mb/s wired connection on a powerful PC.
if you open the dev console's network tab you'll see why it was so "sluggish". I really can't say that I've ever seen a website do so much to load so little.
What are the differences between service costs and operating costs? [1]
I'm surprised not to see taxes and other external costs being taken into account in the Retention Margin. Are they pre-subtracted from gross sales as the input of RM ?
To your second point: (business income) taxes are a cash-flow item, but are out of scope for margin analysis, which is a profitability measure, not a cash flow measure.
Maybe not the right question to be asking, but what tools does one use to track these types metrics in 2021 for a small 1-2 person company? Google sheets?
The definition seems identical to the commonly-used term 'contribution margin', i.e. revenue less variable costs. https://en.wikipedia.org/wiki/Contribution_margin