• 13.5 baseline — 0.6 × season 12.9 (20 gm) + 0.4 × last 10 14.3
• −0.3 opponent leak — they concede 152.3/game vs a 155.4 league average
• −3.7 at this ground — averages 9.8 there (25 gm)
• vs them: averages 6.3 over 3 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.