• 21.6 baseline — 0.6 × season 21.4 (18 gm) + 0.4 × last 10 21.8
• −1.4 opponent leak — they concede 345.4/game vs a 369 league average
• −1.7 at this ground — averages 19.9 there (10 gm)
• vs them: averages 22.3 over 11 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.