Email conventions say "write for 8th graders." It's bad advice for most newsletters.
Why 60 beats 80
Newsletter subscribers self-selected. They opted in because they like your topic and probably have domain knowledge. Writing Flesch 80 (6th-grade level) to a subscribed audience reads as condescending, and open rates trail off.
Target by newsletter type
Subject lines are a separate beast. They don't follow readability rules — they follow curiosity and specificity rules. A 6-word subject line with one surprising word ("The number that killed my Friday") outperforms a clear 12-word description.
Technique: the "paragraph every 2 sentences" rule. Email readers scroll on phones. Paragraphs longer than 3 lines on mobile create a wall-of-text bounce. This isn't a readability-formula thing — it's a format thing that makes readability easier.
Test your draft here before sending. Watch for: average sentence length above 18 words (break them up) and passive voice above 15% (rewrite).