• 12.9 baseline — 0.6 × season 13.3 (20 gm) + 0.4 × last 10 12.4
• −0.1 opponent leak — they concede 211.7/game vs a 213.6 league average
• +1.4 at this ground — averages 14.3 there (28 gm)
• vs them: averages 12.0 over 3 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 1: 4 — far outside his usual range; kept in the average, not smoothed away.