• 13.3 baseline — 0.6 × season 12.7 (6 gm) + 0.4 × last 10 14.3
• −1.3 opponent leak — they concede 1519.7/game vs a 1686.6 league average
• +15.8 at this ground — averages 29.1 there (15 gm)
• vs them: averages 22.4 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.