• 13.5 baseline — 0.6 × season 13.6 (13 gm) + 0.4 × last 10 13.4
• +1.0 opponent leak — they concede 166.4/game vs a 155.4 league average
• −6.1 at this ground — averages 7.5 there (17 gm)
• vs them: averages 9.1 over 9 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.