• 21.9 baseline — 0.6 × season 18.7 (20 gm) + 0.4 × last 10 26.7
• −2.8 opponent leak — they concede 475.8/game vs a 544.6 league average
• +13.6 at this ground — averages 35.5 there (22 gm)
• vs them: averages 38.8 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.