• 3.9 baseline — 0.6 × season 4.1 (20 gm) + 0.4 × last 10 3.6
• +0.4 opponent leak — they concede 39/game vs a 35.7 league average
• +0.1 at this ground — averages 4 there (90 gm)
• vs them: averages 5.0 over 8 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.