• 13.3 baseline — 0.6 × season 12.6 (20 gm) + 0.4 × last 10 14.5
• +1.2 opponent leak — they concede 594.5/game vs a 544.6 league average
• −1.9 at this ground — averages 11.4 there (12 gm)
• vs them: averages 13.1 over 7 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.