• 8.2 baseline — 0.6 × season 7.7 (15 gm) + 0.4 × last 10 8.9
• +0.7 opponent leak — they concede 233.1/game vs a 213.6 league average
• +3.4 at this ground — averages 11.6 there (51 gm)
• vs them: averages 12.9 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. Round 13: 18 — far outside his usual range; kept in the average, not smoothed away.