• 16.8 baseline — 0.6 × season 17.1 (14 gm) + 0.4 × last 10 16.4
• −0.5 opponent leak — they concede 357.8/game vs a 369 league average
• −4.6 at this ground — averages 12.3 there (4 gm)
• −1.8 wet forecast — averages 15 in the wet (6 gm)
• vs them: averages 12.0 over 3 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.