• 12.7 baseline — 0.6 × season 13.3 (20 gm) + 0.4 × last 10 11.9
• −0.3 opponent leak — they concede 207.8/game vs a 213.6 league average
• −2.8 at this ground — averages 9.9 there (50 gm)
• vs them: averages 6.8 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.