• 12.8 baseline — 0.6 × season 13.2 (17 gm) + 0.4 × last 10 12.3
• −0.0 opponent leak — they concede 212.8/game vs a 213.6 league average
• −0.8 at this ground — averages 12 there (9 gm)
• vs them: averages 13.5 over 10 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.