• 21.7 baseline — 0.6 × season 21.9 (8 gm) + 0.4 × last 10 21.5
• −0.1 opponent leak — they concede 367.1/game vs a 369 league average
• −2.9 at this ground — averages 18.9 there (35 gm)
• vs them: averages 19.2 over 6 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.