• 3.9 baseline — 0.6 × season 3.9 (10 gm) + 0.4 × last 10 3.9
• −0.0 opponent leak — they concede 93.2/game vs a 93.4 league average
• −0.6 at this ground — averages 3.3 there (12 gm)
• vs them: averages 3.9 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.