• 8.7 baseline — 0.6 × season 8.8 (17 gm) + 0.4 × last 10 8.6
• +0.5 opponent leak — they concede 164.6/game vs a 155.4 league average
• −1.6 at this ground — averages 7.1 there (55 gm)
• vs them: averages 11.0 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.