• 13.4 baseline — 0.6 × season 13.3 (20 gm) + 0.4 × last 10 13.6
• −0.1 opponent leak — they concede 367.1/game vs a 369 league average
• −2.2 at this ground — averages 11.2 there (15 gm)
• vs them: averages 20.0 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.