• 12.2 baseline — 0.6 × season 11.8 (20 gm) + 0.4 × last 10 12.9
• +0.1 opponent leak — they concede 157/game vs a 155.4 league average
• at this ground: too thin to count (1 gm) — shown, adds nothing
• vs them: averages 10.5 over 2 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.