• 1.3 baseline — 0.6 × season 1.2 (17 gm) + 0.4 × last 10 1.3
• −0.3 opponent leak — they concede 9.5/game vs a 12.8 league average
• −0.8 at this ground — averages 0.4 there (9 gm)
• vs them: averages 0.5 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.