• 12.2 baseline — 0.6 × season 12.1 (20 gm) + 0.4 × last 10 12.4
• −0.5 opponent leak — they concede 353.4/game vs a 369 league average
• −2.0 at this ground — averages 10.2 there (34 gm)
• vs them: averages 13.3 over 3 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.