Subscribe for a monthly "computer napkin problem" where you use first-principle thinking to arrive at expectations about systems in a jiffy without writing any code! Examples from past editions:

What is the expected throughput of a single-threaded, in-memory data-store like Redis? Problem 4
How much do you expect to pay for the storage of logs cost for a standard, monolithic 100,000 RPS web application? -- Problem 1
What do you expect the response time for a query that visits 50 database pages with a 80% page cache hit rate to be? -- Problem 2
With the napkin math reference numbers in hand, pen, paper, and some reasonable assumptions (such as average log size or database record size) you should be able to arrive at an approximate answer. The idea is that a monthly cadence should be about enough to keep the skill sharp. This talk is the best primer on the topic.