• 15.2 baseline — 0.6 × season 14.8 (21 gm) + 0.4 × last 10 15.7
• +0.3 opponent leak — they concede 189.9/game vs a 185.9 league average
• +0.1 at this ground — averages 15.3 there (61 gm)
• −1.6 wet forecast — averages 13.6 in the wet (16 gm)
• vs them: averages 15.7 over 13 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.