• 17.8 baseline — 0.6 × season 16.6 (5 gm) + 0.4 × last 10 19.6
• −0.4 opponent leak — they concede 360.2/game vs a 369 league average
• +9.1 at this ground — averages 26.9 there (49 gm)
• vs them: averages 29.7 over 3 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.