• 13.8 baseline — 0.6 × season 15.8 (5 gm) + 0.4 × last 10 10.7
• −0.0 opponent leak — they concede 186/game vs a 186.3 league average
• +0.2 at this ground — averages 14 there (5 gm)
• vs them: averages 7.5 over 4 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.