• 21.4 baseline — 0.6 × season 21.8 (20 gm) + 0.4 × last 10 21.0
• +0.5 opponent leak — they concede 377.9/game vs a 369 league average
• −0.4 at this ground — averages 21 there (25 gm)
• vs them: averages 22.8 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.