• 13.9 baseline — 0.6 × season 15.6 (19 gm) + 0.4 × last 10 11.4
• +1.9 opponent leak — they concede 618.4/game vs a 544.2 league average
• −2.6 at this ground — averages 11.3 there (23 gm)
• vs them: averages 30.0 over 2 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.