• 15.6 baseline — 0.6 × season 15.6 (10 gm) + 0.4 × last 10 15.6
• −1.0 opponent leak — they concede 322.1/game vs a 344.7 league average
• −7.9 at this ground — averages 7.7 there (24 gm)
• vs them: averages 7.5 over 4 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.