• 39.9 baseline — 0.6 × season 37.9 (20 gm) + 0.4 × last 10 43.0
• −0.9 opponent leak — they concede 336.7/game vs a 344.7 league average
• at this ground: too thin to count (1 gm) — shown, adds nothing
• −8.3 wet forecast — averages 31.6 in the wet (8 gm)
• vs them: averages 30.0 over 6 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.