• 20.0 baseline — 0.6 × season 19.7 (12 gm) + 0.4 × last 10 20.5
• +1.4 opponent leak — they concede 368.1/game vs a 344.7 league average
• −3.7 at this ground — averages 16.3 there (22 gm)
• vs them: averages 9.8 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.