• 8.7 baseline — 0.6 × season 8.7 (6 gm) + 0.4 × last 10 8.8
• +0.5 opponent leak — they concede 225.5/game vs a 213.6 league average
• +1.2 at this ground — averages 9.9 there (51 gm)
• vs them: averages 9.4 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.