• 16.2 baseline — 0.6 × season 16.7 (19 gm) + 0.4 × last 10 15.5
• +0.9 opponent leak — they concede 389.2/game vs a 369 league average
• −4.1 at this ground — averages 12.2 there (26 gm)
• −4.2 wet forecast — averages 12 in the wet (4 gm)
• vs them: averages 19.5 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.