• 8.6 baseline — 0.6 × season 8.6 (20 gm) + 0.4 × last 10 8.7
• −0.1 opponent leak — they concede 153.7/game vs a 155.4 league average
• −0.5 at this ground — averages 8.1 there (34 gm)
• vs them: averages 7.9 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.