• 12.3 baseline — 0.6 × season 12.3 (16 gm) + 0.4 × last 10 12.2
• +0.9 opponent leak — they concede 230.1/game vs a 213.6 league average
• +1.0 at this ground — averages 13.2 there (21 gm)
• vs them: averages 12.8 over 12 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.