• 12.8 baseline — 0.6 × season 12.6 (15 gm) + 0.4 × last 10 13.0
• +0.3 opponent leak — they concede 378.4/game vs a 369 league average
• −1.0 at this ground — averages 11.8 there (9 gm)
• vs them: averages 11.7 over 7 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.