• 17.8 baseline — 0.6 × season 17.8 (10 gm) + 0.4 × last 10 17.8
• −0.2 opponent leak — they concede 363.9/game vs a 369 league average
• −0.6 at this ground — averages 17.2 there (12 gm)
• wet forecast: too thin to count (3 gm) — shown, adds nothing
• vs them: averages 17.8 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.