• 18.6 baseline — 0.6 × season 17.5 (19 gm) + 0.4 × last 10 20.2
• −0.6 opponent leak — they concede 357.8/game vs a 369 league average
• −2.1 at this ground — averages 16.5 there (41 gm)
• vs them: averages 17.3 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.