• 12.4 baseline — 0.6 × season 12.1 (18 gm) + 0.4 × last 10 13.0
• +0.9 opponent leak — they concede 166.4/game vs a 155.4 league average
• −2.8 at this ground — averages 9.6 there (46 gm)
• vs them: averages 10.4 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.