• 12.1 baseline — 0.6 × season 12.3 (19 gm) + 0.4 × last 10 11.8
• +0.0 opponent leak — they concede 186/game vs a 185.9 league average
• +0.7 at this ground — averages 12.8 there (30 gm)
• vs them: averages 11.5 over 6 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.