• 18.0 baseline — 0.6 × season 17.3 (19 gm) + 0.4 × last 10 19.2
• +1.0 opponent leak — they concede 389.2/game vs a 369 league average
• −0.3 at this ground — averages 17.8 there (12 gm)
• vs them: averages 17.3 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.