• 3.9 baseline — 0.6 × season 3.9 (18 gm) + 0.4 × last 10 3.8
• −0.2 opponent leak — they concede 53.8/game vs a 56.6 league average
• −0.2 at this ground — averages 3.6 there (30 gm)
• vs them: averages 3.3 over 3 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.