• 18.6 baseline — 0.6 × season 18.7 (9 gm) + 0.4 × last 10 18.4
• −1.2 opponent leak — they concede 345.4/game vs a 369 league average
• +5.9 at this ground — averages 24.5 there (21 gm)
• vs them: averages 21.3 over 10 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.