• 5.1 baseline — 0.6 × season 5.3 (20 gm) + 0.4 × last 10 4.8
• −0.3 opponent leak — they concede 87/game vs a 93.4 league average
• +0.3 at this ground — averages 5.4 there (21 gm)
• vs them: averages 5.0 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.