• 19.8 baseline — 0.6 × season 21.0 (12 gm) + 0.4 × last 10 18.0
• +1.9 opponent leak — they concede 1851.3/game vs a 1687.8 league average
• at this ground: too thin to count (1 gm) — shown, adds nothing
• vs them: averages 20.3 over 3 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.