• 12.7 baseline — 0.6 × season 12.8 (12 gm) + 0.4 × last 10 12.7
• −0.3 opponent leak — they concede 207.8/game vs a 213.6 league average
• −3.2 at this ground — averages 9.6 there (16 gm)
• vs them: averages 9.3 over 4 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.