• 8.5 baseline — 0.6 × season 8.3 (18 gm) + 0.4 × last 10 8.7
• −0.5 opponent leak — they concede 201.8/game vs a 213.6 league average
• +0.1 at this ground — averages 8.5 there (80 gm)
• vs them: averages 8.7 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.