• 4.2 baseline — 0.6 × season 4.5 (20 gm) + 0.4 × last 10 3.9
• −0.2 opponent leak — they concede 53.8/game vs a 56.6 league average
• −0.3 at this ground — averages 3.9 there (50 gm)
• vs them: averages 2.9 over 7 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.