• 1.2 baseline — 0.6 × season 1.2 (20 gm) + 0.4 × last 10 1.2
• −0.4 opponent leak — they concede 16.2/game vs a 23.7 league average
• −0.3 at this ground — averages 0.9 there (42 gm)
• vs them: averages 0.8 over 5 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.