• 21.6 baseline — 0.6 × season 20.1 (16 gm) + 0.4 × last 10 23.8
• −0.3 opponent leak — they concede 340/game vs a 344.9 league average
• +15.7 at this ground — averages 37.3 there (39 gm)
• vs them: averages 39.4 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.