• 3.9 baseline — 0.6 × season 4.0 (20 gm) + 0.4 × last 10 3.8
• −0.3 opponent leak — they concede 87/game vs a 93.4 league average
• −0.8 at this ground — averages 3.2 there (80 gm)
• vs them: averages 3.5 over 13 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.