• 6.1 baseline — 0.6 × season 6.3 (16 gm) + 0.4 × last 10 5.9
• −0.4 opponent leak — they concede 200.3/game vs a 213.6 league average
• +2.1 at this ground — averages 8.3 there (7 gm)
• vs them: averages 8.5 over 10 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.