• 12.1 baseline — 0.6 × season 11.5 (17 gm) + 0.4 × last 10 13.0
• +0.6 opponent leak — they concede 386.5/game vs a 369 league average
• +0.1 at this ground — averages 12.2 there (5 gm)
• vs them: averages 12.0 over 1 game — 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.