• 12.4 baseline — 0.6 × season 12.8 (20 gm) + 0.4 × last 10 11.9
• −0.8 opponent leak — they concede 145.2/game vs a 155.4 league average
• −0.6 at this ground — averages 11.9 there (83 gm)
• vs them: averages 10.9 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.