• 12.3 baseline — 0.6 × season 11.2 (19 gm) + 0.4 × last 10 14.0
• −0.9 opponent leak — they concede 505.5/game vs a 544.2 league average
• −3.6 at this ground — averages 8.7 there (42 gm)
• vs them: averages 7.8 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.