• 18.7 baseline — 0.6 × season 16.6 (20 gm) + 0.4 × last 10 21.8
• −0.3 opponent leak — they concede 1663.5/game vs a 1687.8 league average
• +13.4 at this ground — averages 32.1 there (31 gm)
• vs them: averages 50.7 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.