• 39.5 baseline — 0.6 × season 39.2 (17 gm) + 0.4 × last 10 40.0
• −0.4 opponent leak — they concede 539.6/game vs a 544.6 league average
• −6.0 at this ground — averages 33.5 there (64 gm)
• vs them: averages 42.4 over 8 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.