• 12.0 baseline — 0.6 × season 12.1 (19 gm) + 0.4 × last 10 12.0
• +0.2 opponent leak — they concede 375.7/game vs a 369 league average
• +0.3 at this ground — averages 12.3 there (32 gm)
• vs them: averages 11.5 over 4 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 18: 3 — far outside his usual range; kept in the average, not smoothed away.