• 30.5 baseline — 0.6 × season 31.3 (20 gm) + 0.4 × last 10 29.3
• −1.0 opponent leak — they concede 333.3/game vs a 344.7 league average
• −1.1 at this ground — averages 29.4 there (60 gm)
• +0.3 wet forecast — averages 30.7 in the wet (15 gm)
• vs them: averages 30.0 over 12 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.