• 31.8 baseline — 0.6 × season 31.9 (19 gm) + 0.4 × last 10 31.6
• −2.0 opponent leak — they concede 345.4/game vs a 369 league average
• −9.3 at this ground — averages 22.5 there (48 gm)
• vs them: averages 24.5 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.