• 1.3 baseline — 0.6 × season 1.1 (16 gm) + 0.4 × last 10 1.7
• −0.1 opponent leak — they concede 12/game vs a 12.8 league average
• −1.0 at this ground — averages 0.3 there (10 gm)
• vs them: averages 0.3 over 4 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.