• 18.0 baseline — 0.6 × season 17.6 (20 gm) + 0.4 × last 10 18.7
• +0.4 opponent leak — they concede 377.9/game vs a 369 league average
• −1.6 at this ground — averages 16.4 there (26 gm)
• vs them: averages 15.7 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.