• 13.4 baseline — 0.6 × season 12.7 (18 gm) + 0.4 × last 10 14.5
• −0.9 opponent leak — they concede 345.4/game vs a 369 league average
• −3.6 at this ground — averages 9.9 there (14 gm)
• vs them: averages 11.0 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.