• 4.5 baseline — 0.6 × season 4.4 (18 gm) + 0.4 × last 10 4.6
• −0.8 opponent leak — they concede 26.6/game vs a 32.7 league average
• +0.4 at this ground — averages 4.9 there (18 gm)
• vs them: averages 4.2 over 9 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.