• 12.4 baseline — 0.6 × season 12.4 (17 gm) + 0.4 × last 10 12.5
• +0.6 opponent leak — they concede 386.5/game vs a 369 league average
• −1.8 at this ground — averages 10.7 there (44 gm)
• vs them: averages 10.4 over 5 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.