• 12.2 baseline — 0.6 × season 11.8 (20 gm) + 0.4 × last 10 12.9
• +0.4 opponent leak — they concede 160.7/game vs a 155.4 league average
• −1.7 at this ground — averages 10.5 there (39 gm)
• vs them: averages 9.7 over 6 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.