• 15.3 baseline — 0.6 × season 14.7 (6 gm) + 0.4 × last 10 16.3
• +0.8 opponent leak — they concede 389.2/game vs a 369 league average
• +4.5 at this ground — averages 19.8 there (5 gm)
• vs them: averages 19.7 over 7 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.