• 18.8 baseline — 0.6 × season 18.9 (20 gm) + 0.4 × last 10 18.6
• −1.2 opponent leak — they concede 322.9/game vs a 344.7 league average
• at this ground: too thin to count (3 gm) — shown, adds nothing
• vs them: averages 21.0 over 6 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.