• 17.5 baseline — 0.6 × season 17.4 (20 gm) + 0.4 × last 10 17.5
• +0.0 opponent leak — they concede 213.7/game vs a 213.6 league average
• −5.2 wet forecast — averages 12.3 in the wet (8 gm)
• vs them: averages 12.8 over 9 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.