• 21.1 baseline — 0.6 × season 20.6 (16 gm) + 0.4 × last 10 21.8
• −2.2 opponent leak — they concede 31.6/game vs a 35.3 league average
• +1.2 at this ground — averages 22.3 there (8 gm)

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.