• 21.9 baseline — 0.6 × season 21.9 (20 gm) + 0.4 × last 10 21.8
• −0.9 opponent leak — they concede 353.4/game vs a 369 league average
• −2.3 at this ground — averages 19.6 there (47 gm)
• vs them: averages 19.0 over 4 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.