• 20.7 baseline — 0.6 × season 20.4 (7 gm) + 0.4 × last 10 21.0
• +1.0 opponent leak — they concede 386.5/game vs a 369 league average
• −0.5 at this ground — averages 20.2 there (38 gm)
• vs them: averages 20.3 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.