• 11.9 baseline — 0.6 × season 12.0 (15 gm) + 0.4 × last 10 11.8
• −1.8 opponent leak — they concede 29.9/game vs a 35.3 league average
• −4.9 at this ground — averages 7 there (33 gm)
• vs them: averages 6.1 over 9 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.