• 12.5 baseline — 0.6 × season 12.6 (20 gm) + 0.4 × last 10 12.4
• +1.0 opponent leak — they concede 230.1/game vs a 213.6 league average
• −0.6 at this ground — averages 11.9 there (70 gm)
• vs them: averages 12.4 over 10 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.