• 3.0 baseline — 0.6 × season 3.3 (20 gm) + 0.4 × last 10 2.7
• +0.4 opponent leak — they concede 106.2/game vs a 93.4 league average
• +0.2 at this ground — averages 3.3 there (51 gm)
• −0.3 wet forecast — averages 2.8 in the wet (8 gm)
• vs them: averages 4.2 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.