• 0.8 baseline — 0.6 × season 0.8 (16 gm) + 0.4 × last 10 0.9
• −0.2 opponent leak — they concede 9.6/game vs a 12.2 league average
• +0.5 at this ground — averages 1.4 there (40 gm)
• vs them: averages 1.8 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.