• 4.9 baseline — 0.6 × season 4.9 (18 gm) + 0.4 × last 10 4.9
• −0.5 opponent leak — they concede 50.6/game vs a 56.6 league average
• −1.1 at this ground — averages 3.8 there (18 gm)
• vs them: averages 3.0 over 12 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.