• 8.7 baseline — 0.6 × season 8.7 (13 gm) + 0.4 × last 10 8.6
• −0.1 opponent leak — they concede 211.7/game vs a 213.6 league average
• +0.7 at this ground — averages 9.4 there (23 gm)
• vs them: averages 12.0 over 2 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.