• 11.5 baseline — 0.6 × season 11.4 (20 gm) + 0.4 × last 10 11.5
• +0.3 opponent leak — they concede 377.9/game vs a 369 league average
• +3.0 at this ground — averages 14.5 there (21 gm)
• vs them: averages 13.5 over 4 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.