• 8.9 baseline — 0.6 × season 8.5 (6 gm) + 0.4 × last 10 9.5
• +0.4 opponent leak — they concede 386.2/game vs a 369 league average
• +1.5 at this ground — averages 10.4 there (10 gm)
• vs them: averages 13.0 over 1 game — 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.