• 42.2 baseline — 0.6 × season 39.3 (16 gm) + 0.4 × last 10 46.7
• +0.3 opponent leak — they concede 548.9/game vs a 544.6 league average
• −9.7 at this ground — averages 32.5 there (6 gm)
• vs them: averages 32.0 over 2 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.