• 12.4 baseline — 0.6 × season 12.6 (20 gm) + 0.4 × last 10 12.1
• −0.1 opponent leak — they concede 211.2/game vs a 213.6 league average
• −3.1 at this ground — averages 9.3 there (18 gm)
• vs them: averages 11.9 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.