• 12.7 baseline — 0.6 × season 13.2 (18 gm) + 0.4 × last 10 11.9
• +0.6 opponent leak — they concede 386.2/game vs a 369 league average
• −1.2 at this ground — averages 11.5 there (28 gm)
• vs them: averages 12.5 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.