• 28.6 baseline — 0.6 × season 28.4 (20 gm) + 0.4 × last 10 28.9
• +1.4 opponent leak — they concede 386.5/game vs a 369 league average
• −0.6 at this ground — averages 28 there (48 gm)
• vs them: averages 26.1 over 9 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.