• 12.1 baseline — 0.6 × season 11.9 (14 gm) + 0.4 × last 10 12.4
• −0.3 opponent leak — they concede 181.2/game vs a 185.9 league average
• +1.2 at this ground — averages 13.3 there (19 gm)
• vs them: averages 13.2 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.