• 18.4 baseline — 0.6 × season 18.2 (15 gm) + 0.4 × last 10 18.8
• +0.4 opponent leak — they concede 377.9/game vs a 369 league average
• +2.8 at this ground — averages 21.2 there (9 gm)
• vs them: averages 15.0 over 1 game — 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.