• 12.4 baseline — 0.6 × season 12.5 (15 gm) + 0.4 × last 10 12.2
• −0.4 opponent leak — they concede 357.8/game vs a 369 league average
• +0.6 at this ground — averages 13 there (67 gm)
• vs them: averages 12.0 over 5 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.