• 12.8 baseline — 0.6 × season 12.5 (19 gm) + 0.4 × last 10 13.3
• −0.2 opponent leak — they concede 210.3/game vs a 213.6 league average
• −0.8 at this ground — averages 12.1 there (62 gm)
• vs them: averages 12.1 over 8 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.