• 68.3 baseline — 0.6 × season 65.2 (18 gm) + 0.4 × last 10 72.9
• −8.6 opponent leak — they concede 475.8/game vs a 544.2 league average
• +3.2 at this ground — averages 71.5 there (18 gm)
• vs them: averages 61.3 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.