• 19.8 baseline — 0.6 × season 21.0 (12 gm) + 0.4 × last 10 18.0
• −1.7 opponent leak — they concede 1543/game vs a 1687.8 league average
• +33.8 at this ground — averages 53.6 there (12 gm)
• vs them: averages 65.0 over 2 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.