• 12.2 baseline — 0.6 × season 11.8 (20 gm) + 0.4 × last 10 12.9
• −0.4 opponent leak — they concede 357.8/game vs a 369 league average
• at this ground: too thin to count (1 gm) — shown, adds nothing
• vs them: averages 11.5 over 6 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.