• 20.3 baseline — 0.6 × season 18.8 (6 gm) + 0.4 × last 10 22.4
• −0.3 opponent leak — they concede 364.1/game vs a 369 league average
• −0.9 at this ground — averages 19.3 there (12 gm)
• vs them: averages 20.8 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.