• 3.9 baseline — 0.6 × season 3.8 (13 gm) + 0.4 × last 10 4.0
• +0.6 opponent leak — they concede 108.9/game vs a 93.4 league average
• +1.2 at this ground — averages 5.1 there (40 gm)
• vs them: averages 4.3 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.