• 15.9 baseline — 0.6 × season 15.5 (20 gm) + 0.4 × last 10 16.6
• +0.4 opponent leak — they concede 377.9/game vs a 369 league average
• −2.1 at this ground — averages 13.8 there (36 gm)
• vs them: averages 12.8 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.