• 42.1 baseline — 0.6 × season 41.1 (20 gm) + 0.4 × last 10 43.6
• −3.0 opponent leak — they concede 505.5/game vs a 544.2 league average
• +7.3 at this ground — averages 49.4 there (39 gm)
• vs them: averages 47.2 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.