• 0.9 baseline — 0.6 × season 0.9 (15 gm) + 0.4 × last 10 0.9
• +0.2 opponent leak — they concede 4.6/game vs a 3.9 league average
• −0.2 at this ground — averages 0.7 there (47 gm)
• vs them: averages 0.9 over 7 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: 4 — far outside his usual range; kept in the average, not smoothed away.