• 17.8 baseline — 0.6 × season 17.4 (20 gm) + 0.4 × last 10 18.5
• −0.3 opponent leak — they concede 210.3/game vs a 213.6 league average
• −4.2 at this ground — averages 13.6 there (74 gm)
• vs them: averages 14.9 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.