• 4.1 baseline — 0.6 × season 3.6 (5 gm) + 0.4 × last 10 4.9
• −0.2 opponent leak — they concede 88/game vs a 93.4 league average
• −0.1 at this ground — averages 4 there (6 gm)
• vs them: averages 4.3 over 7 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.