• 20.8 baseline — 0.6 × season 20.6 (20 gm) + 0.4 × last 10 21.0
• −0.3 opponent leak — they concede 363.9/game vs a 369 league average
• +2.6 at this ground — averages 23.3 there (50 gm)
• vs them: averages 25.1 over 12 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.