• 39.9 baseline — 0.6 × season 36.1 (20 gm) + 0.4 × last 10 45.6
• +1.0 opponent leak — they concede 559.8/game vs a 545.7 league average
• −10.0 at this ground — averages 29.9 there (43 gm)
• −7.9 wet forecast — averages 32 in the wet (12 gm)
• vs them: averages 33.0 over 7 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.