• 20.3 baseline — 0.6 × season 21.1 (18 gm) + 0.4 × last 10 19.2
• −0.3 opponent leak — they concede 363.9/game vs a 369 league average
• −1.9 at this ground — averages 18.4 there (16 gm)
• −1.1 wet forecast — averages 19.3 in the wet (8 gm)
• vs them: averages 15.1 over 9 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.