• 20.7 baseline — 0.6 × season 20.6 (20 gm) + 0.4 × last 10 20.8
• +0.5 opponent leak — they concede 378.4/game vs a 369 league average
• −2.5 at this ground — averages 18.2 there (41 gm)
• vs them: averages 16.0 over 3 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.