• 15.8 baseline — 0.6 × season 15.9 (16 gm) + 0.4 × last 10 15.7
• −0.2 opponent leak — they concede 363.9/game vs a 369 league average
• −2.3 at this ground — averages 13.5 there (65 gm)
• vs them: averages 13.8 over 11 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.