• 17.7 baseline — 0.6 × season 17.2 (19 gm) + 0.4 × last 10 18.6
• −0.5 opponent leak — they concede 207.8/game vs a 213.6 league average
• −4.7 at this ground — averages 13 there (67 gm)
• vs them: averages 11.4 over 7 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.