• 20.0 baseline — 0.6 × season 21.1 (13 gm) + 0.4 × last 10 18.5
• −0.3 opponent leak — they concede 1663.5/game vs a 1687.8 league average
• +20.0 at this ground — averages 40 there (16 gm)
• vs them: averages 33.0 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.