• 3.9 baseline — 0.6 × season 4.0 (20 gm) + 0.4 × last 10 3.8
• −0.2 opponent leak — they concede 89.3/game vs a 93.4 league average
• +0.6 at this ground — averages 4.5 there (31 gm)
• vs them: averages 4.7 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.