• 17.8 baseline — 0.6 × season 17.9 (12 gm) + 0.4 × last 10 17.7
• −0.3 opponent leak — they concede 210.3/game vs a 213.6 league average
• −5.8 at this ground — averages 12 there (9 gm)
• vs them: averages 8.3 over 3 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.