• 12.2 baseline — 0.6 × season 11.6 (19 gm) + 0.4 × last 10 13.0
• +0.6 opponent leak — they concede 224.7/game vs a 213.6 league average
• −1.7 at this ground — averages 10.5 there (6 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.