• 13.5 baseline — 0.6 × season 13.6 (20 gm) + 0.4 × last 10 13.5
• −0.4 opponent leak — they concede 207.8/game vs a 213.6 league average
• +1.1 at this ground — averages 14.6 there (83 gm)
• vs them: averages 14.1 over 8 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.