• 21.5 baseline — 0.6 × season 20.3 (20 gm) + 0.4 × last 10 23.2
• +2.1 opponent leak — they concede 598.3/game vs a 544.2 league average
• +7.5 at this ground — averages 29 there (19 gm)
• vs them: averages 28.4 over 9 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.