• 13.8 baseline — 0.6 × season 13.9 (15 gm) + 0.4 × last 10 13.7
• +0.7 opponent leak — they concede 224.7/game vs a 213.6 league average
• −0.8 at this ground — averages 13 there (92 gm)
• vs them: averages 13.1 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.