• 12.8 baseline — 0.6 × season 13.1 (19 gm) + 0.4 × last 10 12.3
• −0.3 opponent leak — they concede 360.2/game vs a 369 league average
• −0.1 at this ground — averages 12.6 there (52 gm)
• vs them: averages 9.5 over 4 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.