• 4.9 baseline — 0.6 × season 4.9 (20 gm) + 0.4 × last 10 4.8
• +0.7 opponent leak — they concede 64.3/game vs a 56.6 league average
• +0.9 at this ground — averages 5.8 there (4 gm)
• +1.0 wet forecast — averages 5.8 in the wet (19 gm)
• vs them: averages 5.3 over 9 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.