• 20.6 baseline — 0.6 × season 20.3 (20 gm) + 0.4 × last 10 21.1
• −0.3 opponent leak — they concede 364.1/game vs a 369 league average
• +0.9 at this ground — averages 21.5 there (18 gm)
• vs them: averages 20.6 over 10 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.