• 30.0 baseline — 0.6 × season 30.1 (20 gm) + 0.4 × last 10 29.7
• −0.4 opponent leak — they concede 363.9/game vs a 369 league average
• −0.8 at this ground — averages 29.1 there (86 gm)
• vs them: averages 29.0 over 13 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.