• 17.6 baseline — 0.6 × season 17.6 (20 gm) + 0.4 × last 10 17.6
• +0.4 opponent leak — they concede 378.4/game vs a 369 league average
• −1.6 at this ground — averages 16 there (4 gm)
• vs them: averages 21.0 over 1 game — 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.