• 19.9 baseline — 0.6 × season 20.6 (16 gm) + 0.4 × last 10 18.9
• +0.4 opponent leak — they concede 351.8/game vs a 344.9 league average
• +2.5 at this ground — averages 22.3 there (23 gm)
• vs them: averages 21.2 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.