• 18.6 baseline — 0.6 × season 18.8 (20 gm) + 0.4 × last 10 18.2
• −1.2 opponent leak — they concede 345.4/game vs a 369 league average
• −0.5 at this ground — averages 18.1 there (10 gm)
• vs them: averages 16.3 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.