• 20.6 baseline — 0.6 × season 20.5 (19 gm) + 0.4 × last 10 20.9
• −0.6 opponent leak — they concede 357.8/game vs a 369 league average
• +1.7 at this ground — averages 22.3 there (89 gm)
• vs them: averages 21.6 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 4: 10 — far outside his usual range; kept in the average, not smoothed away.