• 4.1 baseline — 0.6 × season 4.0 (20 gm) + 0.4 × last 10 4.2
• −0.3 opponent leak — they concede 87/game vs a 93.4 league average
• −0.6 at this ground — averages 3.5 there (4 gm)
• −1.6 wet forecast — averages 2.5 in the wet (12 gm)
• vs them: averages 3.0 over 7 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.