• 13.5 baseline — 0.6 × season 13.4 (20 gm) + 0.4 × last 10 13.7
• −0.3 opponent leak — they concede 181.6/game vs a 185.9 league average
• −1.1 at this ground — averages 12.4 there (40 gm)
• vs them: averages 15.3 over 3 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.