• 13.1 baseline — 0.6 × season 12.9 (19 gm) + 0.4 × last 10 13.4
• −0.1 opponent leak — they concede 153.7/game vs a 155.4 league average
• −1.1 at this ground — averages 12.1 there (33 gm)
• vs them: averages 12.3 over 10 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.