• 12.6 baseline — 0.6 × season 12.3 (20 gm) + 0.4 × last 10 13.1
• −0.8 opponent leak — they concede 200.8/game vs a 213.6 league average
• at this ground: too thin to count (2 gm) — shown, adds nothing
• vs them: averages 9.8 over 8 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.