• 11.9 baseline — 0.6 × season 11.9 (13 gm) + 0.4 × last 10 11.9
• −0.2 opponent leak — they concede 209.2/game vs a 213.6 league average
• −1.0 at this ground — averages 10.9 there (37 gm)
• vs them: averages 9.8 over 5 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.