• 12.7 baseline — 0.6 × season 12.9 (18 gm) + 0.4 × last 10 12.3
• +1.2 opponent leak — they concede 233.1/game vs a 213.6 league average
• −2.1 at this ground — averages 10.6 there (20 gm)
• −5.9 wet forecast — averages 6.7 in the wet (7 gm)
• vs them: averages 6.2 over 5 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.