• 8.3 baseline — 0.6 × season 8.7 (17 gm) + 0.4 × last 10 7.7
• −0.5 opponent leak — they concede 201.8/game vs a 213.6 league average
• −2.1 at this ground — averages 6.3 there (4 gm)
• vs them: averages 8.0 over 4 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.