• 1.8 baseline — 0.6 × season 1.7 (20 gm) + 0.4 × last 10 1.9
• −0.2 opponent leak — they concede 80.8/game vs a 93.4 league average
• +0.1 at this ground — averages 1.9 there (21 gm)
• vs them: averages 0.5 over 2 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 21: 5 — far outside his usual range; kept in the average, not smoothed away.