• 76.8 baseline — 0.6 × season 74.6 (20 gm) + 0.4 × last 10 80.2
• +1.3 opponent leak — they concede 1715.2/game vs a 1686.6 league average
• +3.9 at this ground — averages 80.8 there (4 gm)
• −30.3 wet forecast — averages 46.5 in the wet (4 gm)
• vs them: averages 80.6 over 8 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.