• 19.6 baseline — 0.6 × season 19.3 (12 gm) + 0.4 × last 10 20.0
• +1.2 opponent leak — they concede 365.4/game vs a 344.7 league average
• +1.2 at this ground — averages 20.8 there (34 gm)
• vs them: averages 17.5 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.