• 5.4 baseline — 0.6 × season 5.2 (13 gm) + 0.4 × last 10 5.7
• −0.6 opponent leak — they concede 82.5/game vs a 93.4 league average
• −2.4 at this ground — averages 3 there (4 gm)
• −2.3 wet forecast — averages 3.1 in the wet (14 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.