• 54.3 baseline — 0.6 × season 61.6 (17 gm) + 0.4 × last 10 43.3
• −6.0 opponent leak — they concede 1504.1/game vs a 1690.9 league average
• −21.6 at this ground — averages 32.8 there (4 gm)
• vs them: averages 88.2 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.