• 18.1 baseline — 0.6 × season 17.9 (20 gm) + 0.4 × last 10 18.3
• −0.6 opponent leak — they concede 357.8/game vs a 369 league average
• at this ground: too thin to count (2 gm) — shown, adds nothing
• vs them: averages 13.8 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.