• 1.3 baseline — 0.6 × season 1.3 (16 gm) + 0.4 × last 10 1.3
• +0.3 opponent leak — they concede 15.6/game vs a 12.8 league average
• +0.0 at this ground — averages 1.3 there (13 gm)
• −0.9 wet forecast — averages 0.4 in the wet (8 gm)
• vs them: averages 0.8 over 4 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.