• 27.9 baseline — 0.6 × season 26.9 (17 gm) + 0.4 × last 10 29.4
• −1.8 opponent leak — they concede 345.4/game vs a 369 league average
• −3.1 at this ground — averages 24.9 there (77 gm)
• vs them: averages 21.3 over 10 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.