• 20.7 baseline — 0.6 × season 20.1 (20 gm) + 0.4 × last 10 21.5
• +1.0 opponent leak — they concede 386.2/game vs a 369 league average
• −0.2 at this ground — averages 20.4 there (36 gm)
• vs them: averages 21.4 over 5 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.