• 13.8 baseline — 0.6 × season 13.9 (15 gm) + 0.4 × last 10 13.7
• −0.8 opponent leak — they concede 201.8/game vs a 213.6 league average
• +1.2 at this ground — averages 15 there (8 gm)
• vs them: averages 12.5 over 6 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.