• 21.3 baseline — 0.6 × season 18.8 (4 gm) + 0.4 × last 10 25.0
• +0.4 opponent leak — they concede 35.9/game vs a 35.3 league average
• +1.8 at this ground — averages 23.1 there (18 gm)
• vs them: averages 26.0 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.