• 12.8 baseline — 0.6 × season 12.9 (17 gm) + 0.4 × last 10 12.6
• −0.9 opponent leak — they concede 343.2/game vs a 369 league average
• −1.8 at this ground — averages 11 there (4 gm)
• vs them: averages 12.3 over 4 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.