• 13.8 baseline — 0.6 × season 14.4 (18 gm) + 0.4 × last 10 13.0
• +0.3 opponent leak — they concede 377.9/game vs a 369 league average
• +0.4 at this ground — averages 14.2 there (10 gm)
• vs them: averages 17.0 over 1 game — 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.