• 12.1 baseline — 0.6 × season 12.5 (8 gm) + 0.4 × last 10 11.6
• −0.1 opponent leak — they concede 211.7/game vs a 213.6 league average
• at this ground: too thin to count (2 gm) — shown, adds nothing
• −0.5 wet forecast — averages 11.6 in the wet (10 gm)
• vs them: averages 9.0 over 5 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.