• 12.7 baseline — 0.6 × season 13.3 (19 gm) + 0.4 × last 10 11.9
• −0.5 opponent leak — they concede 353.4/game vs a 369 league average
• −2.1 at this ground — averages 10.6 there (67 gm)
• vs them: averages 10.2 over 5 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.