• 12.8 baseline — 0.6 × season 12.8 (18 gm) + 0.4 × last 10 12.7
• −0.2 opponent leak — they concede 364.1/game vs a 369 league average
• −2.6 at this ground — averages 10.2 there (16 gm)
• vs them: averages 12.2 over 9 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.