• 17.1 baseline — 0.6 × season 16.6 (20 gm) + 0.4 × last 10 18.0
• +0.9 opponent leak — they concede 224.7/game vs a 213.6 league average
• −1.0 at this ground — averages 16.1 there (16 gm)
• vs them: averages 12.2 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.