• 18.1 baseline — 0.6 × season 18.5 (20 gm) + 0.4 × last 10 17.6
• +0.8 opponent leak — they concede 386.2/game vs a 369 league average
• +2.4 at this ground — averages 20.5 there (79 gm)
• vs them: averages 22.9 over 9 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.