• 15.0 baseline — 0.6 × season 15.3 (18 gm) + 0.4 × last 10 14.6
• +1.4 opponent leak — they concede 233.1/game vs a 213.6 league average
• +2.8 at this ground — averages 17.8 there (58 gm)
• +1.4 wet forecast — averages 16.5 in the wet (15 gm)
• vs them: averages 18.3 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.