• 4.2 baseline — 0.6 × season 4.3 (15 gm) + 0.4 × last 10 4.1
• −0.5 opponent leak — they concede 83/game vs a 93.4 league average
• +0.3 at this ground — averages 4.5 there (33 gm)
• vs them: averages 4.0 over 9 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.