• 27.1 baseline — 0.6 × season 26.9 (18 gm) + 0.4 × last 10 27.3
• +2.2 opponent leak — they concede 399.5/game vs a 369 league average
• −9.5 at this ground — averages 17.6 there (18 gm)
• vs them: averages 18.6 over 12 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.