• 20.6 baseline — 0.6 × season 19.9 (20 gm) + 0.4 × last 10 21.8
• −0.1 opponent leak — they concede 367.1/game vs a 369 league average
• at this ground: too thin to count (3 gm) — shown, adds nothing
• vs them: averages 16.0 over 10 games — evidence, never summed

Derived, not measured — plain arithmetic over his real games, shown in full. Adjustments can overlap (a game at this ground can also be a wet one); they add simply, and this note is the admission.