• 38.5 baseline — 0.6 × season 39.0 (20 gm) + 0.4 × last 10 37.8
• +0.5 opponent leak — they concede 349.4/game vs a 344.9 league average
• at this ground: too thin to count (3 gm) — shown, adds nothing
• vs them: averages 40.0 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.