• 15.3 baseline — 0.6 × season 15.2 (17 gm) + 0.4 × last 10 15.4
• −0.2 opponent leak — they concede 363.9/game vs a 369 league average
• −1.8 at this ground — averages 13.5 there (87 gm)
• vs them: averages 15.2 over 12 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.