• 15.1 baseline — 0.6 × season 14.8 (20 gm) + 0.4 × last 10 15.6
• +0.3 opponent leak — they concede 375.7/game vs a 369 league average
• −2.2 at this ground — averages 12.9 there (68 gm)
• vs them: averages 12.9 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.