• 15.1 baseline — 0.6 × season 15.3 (20 gm) + 0.4 × last 10 14.7
• −0.1 opponent leak — they concede 211.7/game vs a 213.6 league average
• +0.6 at this ground — averages 15.7 there (40 gm)
• vs them: averages 13.6 over 5 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.