• 12.8 baseline — 0.6 × season 11.4 (17 gm) + 0.4 × last 10 14.8
• +0.9 opponent leak — they concede 368.1/game vs a 344.7 league average
• −2.1 at this ground — averages 10.7 there (51 gm)
• vs them: averages 13.3 over 13 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.