• 26.1 baseline — 0.6 × season 26.3 (20 gm) + 0.4 × last 10 25.8
• −0.2 opponent leak — they concede 365.8/game vs a 369 league average
• −8.0 wet forecast — averages 18.1 in the wet (8 gm)
• vs them: averages 18.7 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.