• 17.0 baseline — 0.6 × season 17.7 (19 gm) + 0.4 × last 10 16.0
• −0.1 opponent leak — they concede 365.8/game vs a 369 league average
• −1.8 wet forecast — averages 15.3 in the wet (4 gm)
• vs them: averages 15.5 over 4 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.