• 12.7 baseline — 0.6 × season 12.6 (20 gm) + 0.4 × last 10 12.9
• +0.7 opponent leak — they concede 225.5/game vs a 213.6 league average
• −0.6 at this ground — averages 12.2 there (36 gm)
• vs them: averages 12.4 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.