• 20.5 baseline — 0.6 × season 20.2 (19 gm) + 0.4 × last 10 21.0
• −2.6 opponent leak — they concede 475.8/game vs a 544.6 league average
• +1.8 at this ground — averages 22.4 there (28 gm)
• vs them: averages 20.5 over 4 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.