• 5.1 baseline — 0.6 × season 4.6 (19 gm) + 0.4 × last 10 5.7
• −0.1 opponent leak — they concede 153.7/game vs a 155.4 league average
• −0.1 at this ground — averages 4.9 there (14 gm)
• vs them: averages 4.1 over 8 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.