• 8.3 baseline — 0.6 × season 9.1 (19 gm) + 0.4 × last 10 7.2
• −0.3 opponent leak — they concede 180.1/game vs a 186.3 league average
• +4.2 at this ground — averages 12.5 there (4 gm)
• vs them: averages 11.5 over 6 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.