• 12.3 baseline — 0.6 × season 11.2 (19 gm) + 0.4 × last 10 14.0
• +1.0 opponent leak — they concede 590/game vs a 544.2 league average
• +0.6 at this ground — averages 12.9 there (10 gm)
• vs them: averages 10.9 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.