• 17.8 baseline — 0.6 × season 18.5 (13 gm) + 0.4 × last 10 16.9
• −0.4 opponent leak — they concede 360.2/game vs a 369 league average
• +0.5 at this ground — averages 18.4 there (11 gm)
• vs them: averages 13.8 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 17: 4 — far outside his usual range; kept in the average, not smoothed away.