• 15.9 baseline — 0.6 × season 15.9 (10 gm) + 0.4 × last 10 15.9
• +1.5 opponent leak — they concede 594.5/game vs a 544.6 league average
• +0.5 at this ground — averages 16.4 there (18 gm)
• vs them: averages 10.2 over 5 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.