• 13.8 baseline — 0.6 × season 13.1 (20 gm) + 0.4 × last 10 14.8
• −0.8 opponent leak — they concede 201.8/game vs a 213.6 league average
• −4.5 at this ground — averages 9.3 there (66 gm)
• vs them: averages 7.6 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.