• 13.9 baseline — 0.6 × season 13.6 (19 gm) + 0.4 × last 10 14.4
• −0.7 opponent leak — they concede 203.2/game vs a 213.6 league average
• −3.1 at this ground — averages 10.8 there (84 gm)
• vs them: averages 10.4 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.