• 21.7 baseline — 0.6 × season 21.9 (8 gm) + 0.4 × last 10 21.5
• +0.4 opponent leak — they concede 375.7/game vs a 369 league average
• at this ground: too thin to count (2 gm) — shown, adds nothing
• vs them: averages 16.6 over 5 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.