• 3.1 baseline — 0.6 × season 3.3 (19 gm) + 0.4 × last 10 2.8
• −0.2 opponent leak — they concede 30.6/game vs a 32.8 league average
• −0.1 at this ground — averages 3 there (11 gm)
• −1.6 wet forecast — averages 1.5 in the wet (11 gm)
• vs them: averages 2.4 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.