• 21.5 baseline — 0.6 × season 22.1 (19 gm) + 0.4 × last 10 20.6
• +1.2 opponent leak — they concede 389.2/game vs a 369 league average
• −1.5 at this ground — averages 20 there (42 gm)
• vs them: averages 22.5 over 2 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.