• 12.7 baseline — 0.6 × season 12.8 (17 gm) + 0.4 × last 10 12.5
• +0.3 opponent leak — they concede 158.8/game vs a 155.4 league average
• −1.2 at this ground — averages 11.5 there (86 gm)
• vs them: averages 10.1 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.