• 3.6 baseline — 0.6 × season 3.7 (20 gm) + 0.4 × last 10 3.5
• −0.2 opponent leak — they concede 87/game vs a 93.4 league average
• +0.2 at this ground — averages 3.9 there (92 gm)
• vs them: averages 4.5 over 15 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.