• 5.1 baseline — 0.6 × season 5.0 (20 gm) + 0.4 × last 10 5.2
• −0.1 opponent leak — they concede 153.7/game vs a 155.4 league average
• −0.3 at this ground — averages 4.8 there (9 gm)
• vs them: averages 3.0 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.