• 39.9 baseline — 0.6 × season 37.9 (20 gm) + 0.4 × last 10 43.0
• +0.8 opponent leak — they concede 351.7/game vs a 344.9 league average
• −4.2 at this ground — averages 35.8 there (27 gm)
• vs them: averages 28.5 over 4 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.