• 18.9 baseline — 0.6 × season 18.6 (13 gm) + 0.4 × last 10 19.4
• +0.3 opponent leak — they concede 375.7/game vs a 369 league average
• +3.1 at this ground — averages 22 there (4 gm)
• vs them: averages 16.1 over 8 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.