• 6.5 baseline — 0.6 × season 6.5 (13 gm) + 0.4 × last 10 6.5
• +0.6 opponent leak — they concede 233.1/game vs a 213.6 league average
• +3.1 at this ground — averages 9.7 there (18 gm)
• vs them: averages 10.2 over 12 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.