• 21.7 baseline — 0.6 × season 21.5 (20 gm) + 0.4 × last 10 22.0
• −1.5 opponent leak — they concede 343.2/game vs a 369 league average
• −2.0 at this ground — averages 19.7 there (86 gm)
• vs them: averages 18.8 over 9 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.