• 17.7 baseline — 0.6 × season 17.4 (17 gm) + 0.4 × last 10 18.1
• +0.8 opponent leak — they concede 386.2/game vs a 369 league average
• +2.1 at this ground — averages 19.8 there (31 gm)
• vs them: averages 19.6 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.