• 21.0 baseline — 0.6 × season 21.4 (12 gm) + 0.4 × last 10 20.4
• −1.3 opponent leak — they concede 345.4/game vs a 369 league average
• −4.3 at this ground — averages 16.7 there (9 gm)
• vs them: averages 19.3 over 10 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.